This is how i play with Caitlyn, i have watch other builds and rune suggestions and i think this guide might do the job.

+ Attack Speed
+ Attack Dammage
+ a lot of Armor Penetration (without needing to buy last whisper).

Always buy the dorian sword first, it bust you damage and gives you more hp and life steal.
when you can buy another one with lvl 1 boots plux some hp pots.

Concentrate on killing minions until you can afford the B.F Sword, then berserk boots, after that just follow the rest of the item sequence to make a lot of damage and survive with the life steal. As soon as you get infinit edge i highly recomend you to buy Zeal and Vampiric Specter at the same time, that will give you more attack speed and let you recover from battle quicker.

This a very basic guide not intended to get to the top, but to show what it can be done with a different opinion of Cait. Thanks :)