SEARCH THE FORUM BEFORE YOU POST! (Because its been posted many times)
Anyways go to your local Apple Store and tell them you want to return it for 16GB iPhone and they will let your return it and pay for the difference, (being roughly $100). If you go to an AT&T store there is a likley restocking fee and they will not likley have an iPhone readily available. Check your local apple store though. Don't sell it on ebay cause you can get the iPhone for the subsidized price of $300 by returning it.