Unlocked Sprint iPhone 4s to use T-Mobile service.

I am not sure if this is the correct thread for this question if so, may switch threads.

I recently bought a unlocked iPhone 4S that has been unlocked by a Sprint carrier from a friend, I was never a Sprint customer; I want to know how can I transfer in my T-Mobile service to the iPhone 4S, implying I know about the GSM and CDMA differences.

Thanks,

Shem