Do you want to unlock, or debrand?
As a noob (which I still am) I didn't know the difference.... debranding basically lets you do anything with your phone... with the exception of using a different SIM card.... unlocking... sometimes will only let you use a different sim card while leaving all of the operator restrictions in place....

For unlocking read here:
unlocking my k800i - Top Sony Forums

For debranding... read threads concerning how to "flash" the MAIN and FS of your phone, don't forget the custpack either.

Cheers.