AT&T contract & upgrading

I just bought a iPhone 3G S on Apple.com and it asked me the question about if i'm using it for business and it said something about the $30 data plan, but at which point do I sign up for the new 2 year contract? Surely, I have to sign something, not just buy the phone? Never did a online activation, so sorry if this is a stupid question. I did a quick search, and didn't find anything, I apologize if this has been posted already.