Unhappy [Q] Bad EMEI TMobile Samsung Galaxy SIII

Hello fellows;

I am sad to tell you that I purchased an SG SIII from CL. After some time it was locked by tmobile because of bad IMEI. Now I want to solve this problem. I am leaving USA, so I will be using it on international sim. I really don't know about phones. Can you tell me step wise what can I do with it. 1. Flashing?
2. Routing?
Whatelse?
How?