Are you already unlocked? Also, who is your carrier?
If you haven't unlocked yet, you need to do that first. Here
http://forum.ppcgeeks.com/showthread.php?t=61333
Then you can download and flash any of the roms
here following these instructions
http://forum.ppcgeeks.com/showthread.php?t=43266
You can flash back to the factory rom with these
http://forum.ppcgeeks.com/showthread.php?t=42741