Can you put an AT&T SIM card into a TMobile iphone?

I found an iphone 4 at a theme park and it's tmobile but i have an at&t phone. Is there a way to get the phone to work with my network? If so, how can I do that?

You would have to contact at&t and tell them you want the unlock instructions and they will give it to you they seem to be most cooperative in this area

There's no such thing as a tmobile iphone. If this is a gsm iphone, it is an unlocked att phone, so it should work with att sim card.
By the way, it is not a right thing to use a found phone. Take it to the police, they will help to return it to the owner. Also, be aware that the owner has probably reported the phone lost or stolen.