How can i make my at&t Sim card work in used iPhone 3g?!?!? please help!?

i bought a used iphone 3g off of a friend, and when i put my at&t 3g sim card in, i can only use the phone and the internet in wi-fi mode. how can i make the texing and internet work?....the phone is already jailbroken if that helps..Thank You!