How Do iPhones work on AT&T?

My brother just got a new iPhone so he's gonna give me his old one. I don't have a smart now, so will I be able to swap his old phone to my number? How do I do that?

Call AT&T and have them to send you a SIM card for the iphone. They need an IMEI number. You will also be req'd to have a data plan if you don't already have one.