Twitch is one of the most powerful, position reliant glass cannons in the game, letting you burst anyone down with your auto attacks and giving slow and magic damage early. This is the build i use.

The runes are a bit weird ( i know) , but they are the best AD runes i have (bought them for Gang, another hero whom i kick *ss with).

Pretty self explanatory, all the damg modifiers for early game creep last hits, and late game crit uber damg, with tiamat.

Last hitting is a must early game with twitch, failing to do so will result in failing the mid game, which is where a safe playing twitch generally gets to start kicking some ***.

Twitch's base attkspd gain is very good, so by level 4 or so, you should be able to get last hits even if you somehow miss the REAL last hit.

I start with a vamp scepter, just for that sustain in lane, however if you think you may be up against heavy harassers, get a dorans shield or blade (preferably shield).
The next item is tiamat, as with my summoner spell setup, you can literally 2 hit squishies at the tower who are trying to kill it.
Infin edge next for obvious reasons, even if they are stacking armour, you have your tiamat ult combo to do magic damg to them and still kill em all.
Cleaver next for op dps and armour reduction.
GhostBlade for more armpen, ms for running ( which you didn't need in the beginning cause of your slow.) attk speed for general bonus dps and damg.
Bloodthirster to help your damg even more, and for some crazy lifesteal.

I know it's not so detailed and that i didnt really put much work into it, but if anyone reads it, tries it and stuff, maybe ill slowly add more and more detail.

Try it out ffs.

Best of luck playing LOL.