how do i unlock an ATT phone so that i can use it for Tmobile?

im woundering how i can do this? please help.

Hi you can easily unlock your phone from AT&T network provider using codes ..You can purchase code from online providers like TheUnlockArena.com for 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 the phone from AT&T network provider then freely use it with t-mobile or any other GSM sim ...