Twitch is a very powerful champion that if its played right can do some serious damage as well as to give you one of the best experiences in the line of justice.
ABILITIES:
This part might be a bit over-rated but I'm going to include it anyway for those who don't usually use twitch.
Ambush: Twitch hides in the shadows, waiting for a nearby enemy. If he attacks, he will gain bonus attack speed.
After 1.25 seconds, Twitch becomes stealthed for 10/20/30/40/50 seconds. If Twitch attacks a unit while Stealthed, he gains 30/40/50/60/70% attack speed for up to 10 seconds, depending on how long he was stealthed before attacking.
Debilitating Poison - Infects all nearby champions with Debilitating Poison, slowing their movement speed.
Slows nearby enemy champions' move speed by 30%, plus 6% per Deadly Venom they have.
2/2.6/3.2/3.8/4.4 second duration.
Expunge - All nearby enemies secrete Twitch's toxic venoms from their bodies, dealing damage for each stack.
Twitch's attacks become long range, passing through units, and he gains 15/25/35 Attack Damage.
Lasts 12 seconds or 5/6/7 attacks.
Deadly Venom - Twitch's attacks infect the target with Deadly Venom, which deals damage each second, stacks up to 6 times, and lasts up to 6 seconds.
ITEMS:
In my opinion twitch is very easy to build, at the start i like to get a Doran's blade, along with a health pot. this will allow me to do some serious damage at the beginning that will most likelly get you a first blood. secondly i get my Boots of movility, for a good movements speed, from then on you just need to get a phantom dancer infinity blade and a lot of AD items.
TIPS:
First off, you need to harass your opponent as much as possible when laning mid. You will most likely be laning against another carry. All carries are squishy and you will take advantage of this. Hit them as much as you can and run off just a little bit so creeps dont hurt you too much. Once your Expunge is level 2 or 3, you can go in and hit them once or twice, then cast expunge. Watch how their health drops. They will be forced to port back and lose precious levels or will die soon enough. At level 5 you can get your kill. Pretend to port back, press B right behind your creep wave. Sit there for a sec, then walk away and Q, your Ambush. Walk around calmly and place yourself close to or behind your opponent. Open up, you will have a surprise advantage, an AS boost, and you know that your expunge can end him before he can even get close to hurting you. If against teemo watch for his blinding dart, if he hits you with it cast cleanse. He will be confident and think he can kill you, but will die. If exhausted, cast cleanse and /laugh at them.
If done correctly, you have just gotten your first kill and are closer to getting your boots and Recurve Bow.