So i recently bought a used IPhone that was obviously being used by someone else. I bought it off him an the IPhones dont have Sim Cards.

How to i switch it into my name? my Phone Number?

I tried registering on ITunes an plugging the phone into my computer but it didnt do anything and displays the old phone number an information.

Ive heard i have to go to AT&T to unlock it, or Apple store? is this true? anyone done it already?