Unlocking a HTC Prime usually requires a HTC unlock code. An HTC Prime unlocked should work on any GSM phone networks or carrier.
In order to get a HTC Prime unlock code, I would first contact your phone carrier's support line and request one. Sometimes they will give you one for free and in exchange will need your HTC Prime's IMEI number. You can get this number simply by entering *#06# on your phone's keypad and then pressing enter. If you cannot attain a free HTC unlocking code, then you should review some of the unlock code websites I have provided below.
You should now be able to use your unlock code and the steps below to fully unlock your HTC Prime to other carriers. Enjoy!