Buying an iPhone from Craigslist or amazon?

If I bought an iPhone 4s and I already have AT&T what would I have to do to get it on my contract it whatever?

Don't do it. Really. We did it once and got burned with a really bad phone.

Make sure that the phone works and everything first… It is actually really easy to do… The easiest way is to take the phone into the store and tell them… But you can call the AT&T number and they will be happy to assist you… For Verizon I hit three for technical support and tell them that I need to hook up a phone using an existing number… Then they ask you for the MEiD which is located either behind the battery… But an iPhone the info is located inside… They will gladly help you… Just make sure to call AT&T with a different phone then the one that you plan on using.

Not a good idea.

I personally would meet them at the at&t store and have the phone checked right there before buying it.