I think most current T-moblie phones support the internet, I know my I-888 did way back, and so did the phone before it (in 1999) and the one that came after it (nokia) and the two that came after that (I'm hard on phones, I really miss the one I lost in Page AZ though) right up to my current one.
Two things you need to know *99# may well be your internet access number (Using the phone as a modem that is the number you tell your computer to dial). and you will need to subscribe, Price is reasonable (I'm not sure if it is 6, 10 or 20/month)
The phone may be able to connect to your computer via I/R link (Slow and flakey at least for me, but it works) tether (I have a tether but it won't work with my current phone, worked fine with the last two) or bluetooth (Works GREAT with the new phone)
Different tethers for different phones, Tether costs about fifty bucks. Bluetooth may be included in your computer or I use a dongle