Rule 1: DO NOT PLAY VAYNE
Okay then...
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You should really get used to moving between Autos and repositioning after every auto. Keep in mind that not only the opponent ADC but also the opponent support will (possibly) be trying to harass you, and by repositioning you can also move in to harass them without running straight up, wasting time, losing farm etc.
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For a good starting adc I would really go with Ashe. She's pretty good since the rework. Kiting is easy since her new passive makes all her autos apply a slow. Her new Q gives you bonus atk speed when used, something she previously didn't have, and if you stack it enough you can do lots of damage with the flurry of autos, and it strengthens her slow from passive when used. Her W is still an AoE cone slow, her E is now global (you can easily check objectives or check brushes without facechecking), and her Ult still stuns as usual. While she doesn't have much mobility she does have plenty of CC and I personally am most comfortable with her auto attacks, they feel very smooth.
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You don't need to harass when CSing unless you can keep your opponent from getting CS that way. If you play lost of Caitlyn try to always hit your opponent with your passive autos, and try to line up Q so that it hits minions and the opponent. Harass when you can but I would personally prioritize having a higher cs than your opponent over harassing (and sometimes widening that cs gap means harassing to keep your opponent from cs!) It depends mostly on matchup I'd say. If you can easily harass, yeah, go for it. Being able to move between autos can make csing and harassing easier.
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If you are chasing your opponent, definitely kite. As in, auto attack, move, auto. If you just run after them sure you keep up with them but you aren't doing any damage. Kiting with Ashe is easy since your autos slow. However, remember that if you just right click them after an auto you will move into range to auto them again, which will allow them to simply run away. That's why you must manually move between each auto, so you can keep up.
If you are running away, don't kite unless you have an ability that slows/stuns (you should use it then.) After that, just run. With Ashe you can auto/run away because her autos slow. With other champs it's usually easier to just run away. Most ADCs without mobility spells (like Ezreal's E) usually have some sort of CC you can pop down (like Ashes W, Jinx's flame chompers, etc etc) so use those when escaping.
- the safest ADC for early game farming is Caitlyn, however she does tend to fall off a bit later in the game. But yeah, for safe farming, Cait.
Uhhh, cait tips? Sure.
You have range, use it, especially if your opponent is playing an ADC you can harass easily. Hit them with your passive empowered autos, Qs, etc, you can do this while maintaining decent CS and keeping them from CS.
Your range is your greatest advantage. Use it. A lot. Remember, as an ADC your job is not to kill the opponent's fed champ but just do to as much damage with autos as possible, so focus whoever is closest and just auto them. Especially when ahead and well farmed you can do noticable damage. Her ult isn't that good in teamfights so you can usually do more damage by not wasting the time on ulting somebody who will probably have a teammate tank for him and just auto auto auto. Late game with Cait you Q sometimes but mostly auto auto auto and escape with the net.
Cait is a pretty straightforwards ADC and the best way you can contribute is bullying the opponent ADC hard early game and trying to push and end early before you start to fall off/opponent's ADC catches up. You also should probably build this:
Berserkers Greaves
Phantom Dancer
Bloodthirster
Infinity Edge
Last Whisper
Defensive item (Banshees Veil, Guardian Angel, etc)
Most people start Doran's Blade and a potion. I personally go Long Sword just because I have a weird phobia of Doran items, but you should probably do the normal thing and go Doran's.
First back go for probably boots/bloodthirster OR BF sword if you can afford it.
First big item is bloodthirster, followed by either Phantom Dancer of Infinity Edge. Also upgrade boots to berserkers sometimes.
After you have Bloodthirster/Phantom Dancer/Infinity Edge it's time for either the defensive item or last whisper, you may need to get the defensive item early depending on how much you die.
Oh, and usually Q first. whether to get yordle traps or net next depends on how much bush control you want, if your enemy support is blitz/leona/thresh you might want to get the yordle traps to put in bushes to keep them from hiding in them. Basic rule is just be sure to max Q first :p
It can be hard to secure a kill with Cait's ult because of the whole opponent-can-block it thing but use it to go for kills anyways, you can maybe force opponent support to flash in front, maybe they will miss. When you go B use your ult on the opponent even if it won't kill them because it's CD isn't too long and you can maybe do enough damage to make them want to go B as well :3
That's about it, hope this helped
Hope this helped!