Hi guys!
Twitch is one of the most obnoxious and powerful champions. He is very versatile, and he has a very fast fire rate, so he is good as a support. Twitch is someone you have to be very careful with. Why? Because he is extremely squishy. Because of that factor, you have to play him back and make sure to play support. He is a pretty good mid/solo lane champion, but not the best. He also takes a bit of getting used to, but once you've mastered him, he's almost unstoppable.

I take Armor Penetration marks because once his build is finished he has more than 2 basic attacks per second, making each shot extremely valuable. This means that every shot you hit makes a small difference, but over the course of a small engagement, you can have done tons of extra damage.
I take Attack Damage marks so as to be that little bit more powerful. Twitch's basic attacks are nearly the most important aspect of him, so more damage is probably the best mark to take.
I take Attack Speed Glyphs and Quintessences because Twitch is very dependent on attack speed. That means that those runes are the best for him.

I like to stack attack speed after I get a Doran's Blade. Attack speed is the most important aspect of Twitch.
Doran's Blade: A pretty generic item to start with as an AD champ. This will make you a little beefier and give you some more attack damage, but it will be sold later to make room for better items. You'll want to sell it after you get the B.F. sword for the second Black Cleaver.
Berserker's Greaves: These boots will be really helpful, as they are your first item that increases attack speed. It's a good idea to get the boots of speed, then the Dagger.
Zeal & Phantom Dancer: These two items will continue to get your attack speed up, and give you some crit as well. This item is KEY to your success, so make sure you get it early :)
Black Cleaver 1 & 2: The Black Cleaver is a really helpful item because, now that you have attack speed, you need damage to go with that. The Black Cleaver gives you a nice 55 extra damage per attack and 30% attack speed, which is helpful too.
Sword of the Divine: You can switch this item out for others, but I find that this one is the best for it's money, because it will give you a healthy 60% attack speed increase and a good passive, which is 100 magic damage every fourth attack and when activated gives you 30 armor penetration, making your attacks against tanks (mainly) much more useful :)

I use Flash because if he gets stunned or slowed, he will usually die because of how squishy he is. If you use flash, he can get out of sticky situations easier.
I use Ignite because it is a good multipurpose spell. Also, it can help finish off enemies that are running away, which is very helpful.