APN settings in IOS 6? (iphone 4)?

I'm having trouble sending/receiving pictures on my iphone. There's a red exclamation mark when I try to send my pictures and I don't ever receive pics sent to me. I reset the Network Settings but nothing happened. I don't know how to adjust the apn and other settings in order to get the mms to work. The phone is not unlocked though. It's an at&t phone and I'm using an at&t sim. Also, where can I get the APN information for at&t? Thank you!

AT&T's website perhaps?