The sim cards are the same. Take the sim card from your current phone and put it into the iphone (if you haven't already) and then plug the phone into itunes. If that doesn't work, call customer care and have them add the imei of the phone (accessible through the settings menu or on the sim card tray) and the data plan to the number you want to use the phone on. If you've been using the iphone without the data plan, and accessing the internet, be ready for a huge next bill.
And don't go to an AT&T store. They will tell you to take the phone home and plug it into itunes.