Switching a SIM card to an iphone

[smadxx]

My dad got the new iPhone and gave me his old 3G one. I have AT&T and i put the SIM card in and it was working great until i left the house and it kept telling me I didn't have a data plan. On my old phone, I had an unlimited data plan. I'm assuming that the iPhone needs a different plan... How can I get it? Do I have to bring my phone in or can I just do it online somehow?

 

[iSaint]

Call 611 from your phone or go by AT&T. The iPhone data plan is separate.

Have you tried to plug it in to your computer?