Everyone seems to think assassins are zero-effort noob champs. Therefore, I tried playing a few. And I fed horribly. Perhaps it's just the completely different playstyle from my mains (tank main here), but I cannot seem to win a game or even put up a fight when playing as an assassin champion. Can anyone help me figure out what I'm doing wrong?
Assassins take LOTS of practice. But once you get them down, they are freelo at low elo.
Your topic is kind of vague and it depends on the champion you're playing.
Though as an assassin you have to be at the back lines. You let the tanks do the initiating and after a few seconds you run up into the fight and scan out the carry you want to assasinate. Never initiate unless you have 100% chance you'll make it out alive or kill 2 of their squishies. That's the simple part. It may sound selfish but it's better than having a dead assassin in 1 second.
For example, Twitch may not be an assassin but to be a good Twitch you have to act like one. When the tank initiates, that is when you start stealthing (note: sometimes you shouldn't start stealthing before the fight), and after a few seconds people don't really notice or care about Twitch because they are too busy fighting your tanks/fighters/adcs. That's when Twitch (and assassins) strike.
well as a former 
main for the start of the match play passive poke whenever the other person is trying to cs max get w first then e second use E to help escape ganks then get q and max it first max w second and third e then put points into ult whenever you can start building 
first against an ap champion mid if they are beating you get 
then build 
then if the other team is stacking armor get 
if not get 
now since its been awhile since ive had to build for talon but these items are good items to look at when you build when going in if you absolutely know you can reach your target ult out of their sight range then q and cut throat to them if not save ult for when you cut throat to them in team fights wait for everyone on the other team to go in then ult and cut throat to the squishiest champion that deals a lot of damage if you do it right you should be killing the adc every team fight and hopefully winning the game
Timing and cunning man.
The bigger part of assassins is knowing when to dive in and get that kill. The bigger thing about this is knowing when the CC has all been used. Most assassins are very susceptible to CC of any kind, so wait for their stuns/knock ups/fears/taunts before running in.
Make sure your burst (engage, kill, disengage if possible) is capable of killing your target before A) your enemy can respond B) your enemy can take appropriate action (such as cc or murder your face) C) your target can get out of range and (most likely) murder your face
Generally if you can kill them in under 1 second, or create enough of a jumpscare you should be fine. Your trinket is very important so upgrade that to a Greater Stealth Vision Trinket when possible so you can keep an eye on their primary objectives (Dragon, Red, Blue). When an Opportunity presents itself (a wild ashe has appeared!) kill em!
If for whatever reason you find yourself weaker than ALL of the enemy team.... Wait for a teamfight to drag on to about when the opposing team is left with 1/2 or 1/3 hp then clean the mess up for some ez gold to boost your ability to kill things and to "secure" those kills :P
I main assassins and yes they are like polar opposite of tanks lol. As for what your doing wrong i don't know ive never seen you play
Oh, I like this topic.
I have the same problem. I can play Malzahar and surprise them with my ult. But ... He is more a mage than an assassin. Yep, I will try those things: wait for the initiation than engage on the right carry.
On early game, when in lane, I found it quite difficult to disengage without getting hit tough. I think it's my bigest problem right now. It's weakened my early game.
P.S.: Sorry about mistakes, english is not my first language.
Timing, courage, and a whole lotta gusto.
I mean that quite seriously, you gotta know when to go in and spam you abilities. Usually after a good amount of harass. You gotta trust yourself and your champions strengths. I see a lot of people "try" to run in and deal damage but after taking one hit they "try" to run away.
Once team fighting, trust your tank to give you room to escape after diving the back line. Also never go in the front unless you're 
.
They are opposite of tanks completely, you may feel compelled to help poke when a team fight is about to break out, but trust you teammates and wait for the opportunity to burst the enemy carry. If you know you can get a kill, don't be afraid to initiate for your team.
