You start here:
http://forum.ppcgeeks.com/showthread.php?t=32382
The first link in this link is the one you will spend most if not all of your time in. Basically there are 3 major steps that you will have to go through.
1. Unlock Your Phone
2. Update Your Radio
3. Flash a New ROM (There are many roms from several different "chefs". Some roms are made for specific carriers. You would look for an Altell rom. Or You can install a rom and then run the carrier cab for your carrier.)
The Link above is going to be your best resource. However, if you need a question answered, just ask. This site has a great search feature which I suggest you use before asking because someone may have already had that question answered in another post.