Can i activate my no-contact iphone 4 without buying another phone?

I bought an iPhone 4 with no contract, but i'm thinking of "activating" or getting a contract with at and t. Can i use my i phone or do i have to buy another cellphone again?

U def don't want att they suck. My boyfriend and i have both have them. Anyway, no you don't need to buy a new one. As long as it has a sim card then its good. If it was not originally an att phone then you will have to get it unlocked to do so. But thats alot cheaper then buying a new one.