Can you put a T-Mobile micro sim card into an unlocked iphone 4s?

My brother recently gave me his iPhone 4s since he got the new iPhone 5, so I can't make calls, or text, or get on the internet (unless I have Wi-Fi). I found my way into the sim card slot for the iPhone, but my MyTouch 4G sim card is too big.Me and my Mom are going to go the T-Mobile store to get a micro sim card so I can transfer my data onto it, so I can put it in the iPhone. Will it work?

Yes it should work once they know what there doing.

If its unlocked as you said, then yes. Not a problem.