I want to unlock this At&T phone so that I can use my T-mobile sim. Help?

Ya sure only after unlocking the phone from AT&T network lock you can freely use it with t-mobile sim .You can easily unlock your phone using sites like MobileUnlockSolutions.com here they are rendering network unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with your t-mobile sim ...