How to unlock IPhone 4s ATT onto TMOBILE network?

My girl has a iphone 4s firmware 2.0.12 on AT&T's network. She reported it lost and is giving me that phone and she is getting a new one for $200. I want to unlock mine on T-Mobile's service. What all do I need to do in order to unlock it? They can't do anything or trace/lock the iphone once i have unlocked it onto another carrier correct?

There's a website that unlocks it for cheap.
Swagthegame.com