Dorans Bladex3, Attackspeed boots, Black Cleaver, Phantom Dancer, Infinity Edge, Blood Thirsterx2 or Blood Thirster/Last Whisperer depending on enemy team builds. You may also want to consider a Frozen Mallet in there for good measure if your having trouble catching enemies it provides really nice CC and a decent amount of AD with a nice bonus to health.

My runes are
Red/Quint armor pens runes
Yellow mana regen per level runes
Blue CD runes

Masteries, take offensive all the way down 21 points in. Make sure to get the armor pen, attack speed, attack damage, and crit. Then dump the rest of the points in Support for the exp boost.

As Caitlyn you NEED to be mid unless there is a good AP champ on your team whom you are confident can win mid lane and carry the game (which I find rare when I play cait). Caitlyn if played well can not only annihilate ANY mid you could feasibly run against but will also have the tower down by 5min almost every time. If your confident in your abilities to kite and harass with your auto attack start with her trap. If your not used to PROOOOOO kiting then start with her Q(both are good starts I start with Q though) but take note, the more enemies it hits the LESS damage it will do. You want to save your Q for hitting by themselves or before it hits anything else. It does ridiculous damage when you level it. Other than that theres not much to playing Cait. Always trap a little to the left or right of your lane dont trap the bushes directly you want your trap to stop someone in their tracks, never and i mean NEVER use 90 cal net for anything except escaping ganks (which it is the pro'est escape in LoL along with a well placed trap IMO). When you hit 6 and get your ult, look at your bonus ad+ult damage and that will be almost exactly the amount of hp you need to harass your opponent to before you ult them. Other than that theres not much to playing Cait.

IMO Caitlyn is the GREATEST AD ranged carry in the game hands down no questions asked. She can take down any champion 1v1 and at later levels can 2 or 3v1 no problem. Play smart, never overextend when MIAs and dont feel obligated to go out of your way to gank bot or top. Map awareness is literally half of what makes you a good player in LoL. You should always be aware of your enemies habits (constant lane ganks) and keep an eye if one of your lanes is getting pushed back to its tower. If thats the case its time for a gank.

CAIT ES #1