Time for some light reading. Everything you wanted to know about flashing to a custom ROM:
http://forum.ppcgeeks.com/showthread.php?t=43266
How to unlock your TP so you can flash:
http://forum.ppcgeeks.com/showthread.php?t=42503
Do lots of reading first. If you choose to install a custom ROM, just go slowly and follow all instructions completely and you should be fine.