Can you get in using your root password? If you can then you change your password at the command line like this:
passwd username
and that will prompt you to change the password for user username.
root is about the same as administrator, but your have ultimate control over EVERYTHING. Nothing in the OS is bared from you. So you can do stuff like accidently delete the entire OS and stuff like that.
If you can't get in as root, then generally you boot up in single usermode and try to use the passwd command to reset it.
I don't know PPC bootloaders though, only x86. Generally you would type something like "linux-single" or "linux single" or edit the boot up line and add a 1 to the end of it. I think for yellowdog linux-single is the correct boot entry command.
Thats the easy way.
Another method is to boot up with a CD and switch to a command line (alt+ctl+F2) (or any other F button from F1-F6,
Then once you get access to a command line you make a directory (mdkir /hardd) and then mount your OS's root partition to it.
Then you cd into it and issue a "chroot ./" command, once you do that then you issue the passwd command for root to reset that. Then type exit, cd / ; umount /hardd
Then reboot and everything will be fine.