I assume you have a phone subscription including data use with AT&T. Then their customer support might be able to help you.
As a first try: see http://www.cellphonemall.net/wireless/store/custom.aspx?id=4
I just purchased a Samsung GT-S5233A and I can't get the internet or mms messages to work. I have AT&T as my network. I already changed sims card. I can make calls and send text but I can't get the internet working. Thank you!