More often than not I'll be placed against a heavy kill lane bot, and since I'm new to competitively playing ADC, I can't exactly find a nice balance between csing and not getting absolutely demolished while csing. Any tips?
Side question: When and how should I harass? Thanks in advance!
By muting the player who's harassing you, duh.
:P
Tip One: Pick 
She has the longest range of any adc but late-game Tristana, so you can hang as far back as possible and farm.
Tip Two: If you support is stepping forward to harass, go with him. Don't DIVE, of course... but let the support dictate where and when you go for poke. If they're playing passive, you should too.
Tip Three: Remember that a lot of hardcore kill-lanes in bot will fall behind in use if they DON'T get kills. A support with low gold from lack of CS will have a ton of utility and CC effects. A kill support with no gold is going to have trouble killing things as the game goes on. This doesn't mean you shouldn't respect support Annie's mid-late game burst... but without her being fed she'll likely be behind in damage or in durability. Your team should be able to abuse her weak spot.
Tip Four: If your support has a heal, you can afford to eat a little harass while farming, just remember that a heal won't usually save you from a brutal all-in, it's good for shrugging off poke. If you have a Sona/Soraka/Taric or whatever healer don't be afraid to eat a little harassment to get CS. But if you're against a team that is built to just get on your and kill you, like Leona/Graves or the like... you'll want to play it safe.
Tip Five: Which supports are botlane changes the matchup far more than which adc's you pick. Plan how you're gonna play your lane around who you have backing you up and who they have backing them up. Exploit weaknesses in their comp. If their support has no sustain, try and poke them as much as you can without putting yourself at risk. If they're built to dive, let them push the wave to your tower so you have less distance to flee if you get dove on. If they have a blitzcrank, hide behind minions and don't stand in pull range by their tower. Usually the support is more a factor in winning or losing bot than the adc... though not always.
Tip Six: Practice. Practice a lot. ADC is a simple job on paper. Free-farm and then right click. The difficulty of the role is that EVERYONE is trying to kill you. Staying alive usually means more than getting a kill and then dying for it. You need to live as long as possible to push towers after teamfights and to do as much damage as possible over the course of a team fight. Let the tanks, mages and bruisers make the bold plays. Let the support save lives with well-placed CC. Your job is the somewhat bland but game-winning task of plinking out autos as safely as you can for as long as you can.
It falls mostly on the support to be honest... if your support is standing back at your turret while you're trying to cs against an aggressive lane, you're going to get harassed hard and you're likely going to give up first blood if you make any small mistake. Bot lane is really a team effort. If the other team is pushing hard to harass you, your support needs to do their job. Either harass their adc back, or for every 1 hit their support gets on you your support should get 2 off on their support. Or if you've got a Soraka or something then they should be healing the harass back.
As an ADC, you should be focused on last hitting. If the enemy adc is pushing, push back to try to get level 2 and level 3 at about the same time. If the other adc is only last hitting, you should do the same and use the time in between to throw auto attacks at their support or adc when within range (and push otherwise so you get levels 2 and 3 earlier and attempt to make large trades with the level advantage).
All else said though, it is a duo lane and thus you need a support with a clue. I can't begin my complaints about supports I've had some games - just know if the minions are in lane and your support is just standing at the turret you're going to have a bad time (so play defensively and ask the jungler for help).
Great question!
Overall, harass is usually given by whoever is on the offensive. In other words whoever is pushing the lane. This is because in the early levels minion aggro is a serious concern. Especially in the fragile levels of 1-3. What this means is that of you have a lot of minions with you, and you get hit, they take damage as well. Inversely if you try to trade to dissuade harrasment while they have more minions chances are you'll most likely lode the trade.
tl;dr The only 'good' time to take harass if if your lane is pushing, in a general sense.
Harass can also be dissuaded by being aggressive yourself. A low champion will probably not test their luck if you can almost kill them if they try. The key to setting a lane's aggression is the early level 2. The second you get to lane, start killing the minions ASAP (last hitting as well) to push your experience faster. If you get to level 2 before them (you're support should level at the same time as you), you not only have a significant stat boost but you also have an additional ability! Even if you don't get a kill, more than likely you'll knock them down hard enough that they'll have to play timidly until they can shove down pots or their jungler comes to gank. On the other hand you also need to be careful this doesn't happen to you.
tl;dr Push your lane to the level 2 quickly and go ham. A half health enemy will be much less willing to harass you.
And finally it comes down to who you're playing as and against. And with that it comes into auto attack range (or Q range in Ezreal's case). If you're shorter rang than them, chances are you'll just have to last hit where you can until your support can give you an opening (i.e. your support takes a hit so you can toss an auto in before the enemy adc can attack again). If your support can't seem to accomplish that you'll just have to play defensively and wait for the wave to come to you. Practice cs'ing under turret, if you can get that skill down well you'll eliminate most of their advantage.
If you have longer range it's much easier because you can wind up an attack much sooner than they can. Caitlyn has the highest base auto attack range and is the best at this. Kog'maw has a very high range with W active up as well.
tl;dr auto attack range is a huge factor in deciding when to harass. Shorter ranges are more reliant on their support's actions than longer ranged adc's.
Hope that helped!
More often than not I'll be placed against a heavy kill lane bot, and since I'm new to competitively playing ADC, I can't exactly find a nice balance between csing and not getting absolutely demolished while csing. Any tips?
Side question: When and how should I harass? Thanks in advance!
There is only one way to avoid getting harassed while csing: Stay outside of the range of the enemy's attack range or targeted ability or juke the skillshots. In general, this means you should position yourself so you're walking up the minion wave when you're trying to cs and staying outside of the enemy's range so they cant get free harass on you. Sometimes, this may even mean missing out on the cs because you might lose a chunk of your health just for that 1 cs (its not worth it unless you're immediately recalling to buy after or have enough sustain to immediately get back that health)
If you want an easier time csing, you have to control the minion wave better by maintaining a bigger minion wave than the enemy does. By doing this, if the enemy harasses you, they're more likely to take more minion aggro AND since you have more minions, this forces the enemy to spend more of their autoattacks/spells on the minion wave.
If you're trying to decide when you want to harass the enemy, harass the enemy when they go in for cs. Pay attention to the hp bars of your minions. When one is low hp, thats a indicator that the enemy adc will walk up to cs it. You trade with them in this scenario because it forces the adc to choose between:
Source: I'm a diamond ADC main. Its my focus to understand the lane mechanics and mind games to help me win laning phase against higher elo players.
Assuming me and the other duo lane comp is equally matched range/damage/skill wise the first thing I usually do when me and my opponents get to lane level 1 is start a slow push. I call slow push when you slightly damage the creeps more than your opponent and maintain a slight exp/minion advantage. This makes trades easier and it gives you windows of opportunities to abuse a level advantage. Catching your opponent out when you're above them a level can win you the 2v2 lane early.
The faster you push the bigger your temporary advantage is. However your slow push ends sooner because your creeps will overpower theirs and eventually crash into their tower. Pushing too fast gives your opponents free farm usually. Just don't instant delete waves and you should have advantage in trades. And ward of course.
When you have a range advantage, for example you're playing Caitlyn, you have more options than straight up outplaying your opponent in trades. Instead of pushing minions at all you can just hit the other AD when he CS's out of his range. This is will push the lane for you because you'll get creep aggro while making better use of your range advantage to damage your opponents. Land a few solid hits and your lane will probably start slow pushing. Punish their attempts to last hit.
It's also viable as Caitlyn to push stupidly hard with Q and harass them while they farm under tower. Probably best when your support also does poke and you've got them low. If your support is all in type then maybe it's better to reset lane and freeze. Or tower dive if possible.
When you have health/summoner advantage but your opponent decides he's not ready to run back to base you wanna freeze and zone them. You can get a better freeze after a lane reset by using your champion's body to make enemy melee creeps form a row. You run into their melee creeps for a sec then run behind your creep in front. They'll all focus your first minion and your lane will push back towards you even while you last hit. You might see this in LCS lane swap scenarios. Korean teams have been doing this for longer.
This knowledge could help you mid/top too when you're playing with a range advantage (mostly ranged tops, because mid lane is short with more emphasis on roaming, and spells do not draw creep aggro)
When I supported I always hated that my ADC never took advantage of openings created for kills and free damage when I bullied the enemy. The ones that did meshed really well with me and we'd dominate the lane hard. As a support I also know some champs require more babysitting (trist)
When I started playing ADC I was very aggressive as well as trying to last hit. I quickly realized that unless my support set something up I'd get demolished.
So what I learned is that the support dictates the bottom lane and how you play it. You have to follow the supports lead not the other way around. If your support wants to babysit you just take CS and chill. If your support is all over the enemy try to back their harass with shots of your own, you'll quickly find the enemy zoned or just in a really bad spot. The short of it is you HAVE to let the support dictate your lane, if you try to and they're not on board with you you're going to get dominated.
Harass them back and position in ways that make them have to go out of their way to try to hit you, which puts THEM out of position.
If you aren't being aggressive in lane, you're voluntarily throwing away your lane presence and basically putting up a sign that says "Please, destroy me". Which people will gladly do.
From what I have seen as a support main, let your support poke, to get them to move, and then you poke. I use 
and 
frequently. And I use Morgs W as area denial to force them out of position while helping bring down minion health. Or Lux' E for the same purpose. A timid support can be fatal for a bot lane. People sometimes pick support because they feel it is the least important role and don't want too much pressure on them. I learned early on that a good support can dictate the effectiveness of the ADC.
As someone else said, if your support IS timid for whatever reason, follow their lead, and perhaps you can get a surprise kill by your tower since you will probably be pushed. Also, if they have you pushed to your turret and back off while they have health/mana, dont follow, its most likely a setup for a gank. lol
Also, keep moving. Constantly. And stay behind your minions as a lot of your enemy supports utility needs a direct LoS, like Morg, Blitz, Thresh Q.
If you have a ranged support like 
and they aren't using autos to poke, that is either a sign or forgetfulness, or timidness lol. Encourage them to auto poke. Two people poking is better than one.
First it depends entirely on the matchup. If they have better early kill potential then don't trade with them early on unless you know 100% that you can get away with it. Keep creep wave closer to your tower until you're able to fight.. which after a few levels you should get a good idea of your damage vs their damage. If you're able to get early harass on them before they try to trade back, and they don't have a support with a heal, then always keep their health lower than yours by constantly trading. A lot of your success in trades really requires a competent support. Same with csing to an extent.. if your support does nothing then you can be zoned out of cs easily. in that case just tower hug and do your best to farm under tower until a gank.
What I see happen a lot is a bot lane will go into lane and let themselves be zoned out of cs, because they got out traded those first few levels and only had 1 potion and don't want to back so early. Doing some early damage can really work in your favor for early laning.
Really my best advice to trading bot lane is to not take any damage for free. But again, this really comes down to your support.
Against a kill lane I would push them to their turret and poke them because minion aggro will win most fights early on, try not to let them get level 2 first, and time harass with when they go to CS. Also don't push without wards, if you can poke them fairly low call for a gank from your jungler or mid and dive them. Also if you hug the far wall from the turret you can get closer without drawing turret aggro, probably shouldn't do that against a Thresh/Blitz though.
depends on the matchup in a lot of cases.
general rules:
try to auto them or throw abilities between your CS while they're attempting to last hit.
try to hit them back whenever they move forward to auto you.
be wary of support positioning and dont let them sandwich you.
