How to switch accounts on iphone 4?

My friend just gave me his iphone 4 because he was changing networks. I get it and I try to take the sim card out and it is a different size and the phone has no service. How do I put this phone on my account if I can't put my sim card in? Do I need to take it to AT&T and tell them to get me a micro sim card? Thanks if I sound stupid.