The first step is to plug it into craigslist. Make sure you have a reasonable price installed. Ensure that when you meet with the buyer, you take the money and your SIM with you before logging out. Then you will have to install the money somewhere that can sell you a compatible phone. Finally you'll have to put your SIM in the new phone and turn it on.
Thanks for doing this. This was the only reason that I was wanting to get this phone. If I cannot unlock the phone and use it on 3g on at&t, then it is not worth it (I already have a t-mobile phone that works on at&t and one that works on edge).