How can i use the internet on my unlocked g1 with a prepaid t- mobile sim card?

i just unlocked my g1 by using an AT&T sim card and puting the unlock code given to me from the t mobile costumer service, my prepaid t mobile sim card works fine but i cant go on the internet nor login my Google account. how can i change the internet setting so i can surf the net on my unlocked G1?