Go to your Settings, Wireless&Networks. Then maybe Mobile Networks. Select Access Point Names. , and see if it picks up the APN (access point name)
For example, mine says beeline.internet.ru (as the default.
If there is nothing there, you can select Menu, New APN.
You will need to know the apn address for T Mobile.
It will probably be something like tmobile.internet.com but you should check with TMobile.
You can also check (while you are there in the menus) and make sure you have Data Enabled in your Mobile Network settings.
Understand that different devices have different menu settings, but you should be able to look around and find them.
Sorry the other guy was kinda short. sometimes people forget that they too were once a n00b, and had to ask.
Welcome to AC by the way !
EDIT: And one more thing, make sure you do have a data plan with TMobile. We don't use them over here, as only corporate users go with carriers. So I'm a little ignorant on what data plans they use in the states.