Against certain champions, especially champions with a lot of poke, Twitch's beginning can be difficult. But mind that he can easily outtrade in a 1v1 fight in the early game and can still do a number on a champion in a 1v2 because a good few stacks of E really can not be traded against. If you are allowed to be aggressive, feel free to risk some going invisible to either tear away at health or get kills, but never be too far away from your support unless you know what you are doing. Also, do not be afraid to 1v1. As long as you get off your E at three+ stacks, you should win the trade in the early game.

Twitch's mid game will probably be like how you did in the beginning, unless, of course, it was a rather uneventful beginning. With Twitch, things can really get crazy, so, all at one time, you must be careful enough to live and yet dangerous enough to get risky kills.

You can feel free to split push seeing as to how BotRK, IE, and Berserker's Greaves will make wave clear pretty easy and you should definitely aim to take any towers you can by Q'ing for extra attack speed and trying to have your teammates pressure other lanes and/or tease 4v5 team fights that they actually don't intend to go through with. You can snoop around the jungle. You can 1v1 a good number of champs (depending on how fed they are, of course), and you can even team fight. Twitch's mid game isn't a drop off and is rather limitless, I think, but you should really aim to get big here so that your itemization is absolutely dominant and your late game is easy.

Even an under-fed Twitch can still be pretty effective here. A fed Twitch, however, can rip through anything. Take advantage of your itemization and your R by frequently engaging in team fights. Twitch excels in team fights as long as his position is correct. As with any ADC, you must, of course, not be scared of attacking turrets. Too many times do people simply stand at the steps of a base trading pokes even though minions are going through. If you attempt turrets and force a team fight but get into correct position, you are doing yourself a favor.

Item-wise, anyone who has not built armor will be crushed and those who have are hopefully being peeled by your team. The speed of the attack and the damage, even without a Last Whisper, can make Twitch a nightmare for tanks even with huge armor. So just make sure that you're keeping correct positioning in team fights and shoot away.

When it comes to roaming, few are better than Twitch. Don't be too scared to try to steal buffs and solo the Dragon because it'll be quick and you can get away easy. Wards, of course, will still be heavily relied upon, though, so encourage your teammates to place wards as you build without using money on anything except your items of mass destruction.

Twitch's usefulness in team fights always comes down to his positioning and itemization. Position-wise, you'll probably want to be only a bit behind your front line so that you can tear away at the tank if you think you should and/or so that your R allows you to reach the back line and shred any squishies to death. Don't be afraid to focus the tank if the squishies are directly behind the tank and your R is up. This way, you take down the front line and put massive damage on the back line.

With Twitch, your immediate get away ability is non-existent. Twitch relies on a short period of freedom to get away, but with no peel, that won't happen. A team comp heavy with crowd control is really nice, so just stay at a safe but you-can-damage-champions position and you should be fine as long as your team peels.

Pros: The all-out damage Twitch can just shred through anything, armor or not. Come late game, you can 1v1 most any other ADC and it's almost an assured victory in the 1v1 if you get the Q off successfully.

In teamfights, good positioning will allow you freedom of target focus and great damage.

The ability to sneak around and actually get things done is fantastic.

You can take over a lane for a teammate for a short while and feel pretty safe doing it.

Punishing failed ganks is extremely easy.

HUGE damage.

Cons: Absolutely needs an aggressive support who can either damage a lot of basically shut down champions by not allowing them to move quickly or even at all.

Gets destroyed with bad positioning and no peel.

Can be mana hungry in the early game.

Requires some time of playing to learn when/how to go in and when/how to get out.