You need to get into single use mode for steps one and two that are listed below.
This page will tell you how to get into single user mode.
http://support.apple.com/kb/HT1492
Basically, you hold down the command + s key then powering on your machine. The command key has a little apple symbol on the lower left. It is between the alt/option key and the space bar. On a PC keyboard, it will be the windows key, I think.
1) ( Do you know Unix. You are in a Unix single user console. ) The setup commands you need should be listed on the screen. For Mac OS 10.4.11, the commands are:
# follow whatever your startup says. This is what I have.
# Type the follow two instructions to access the startup disk in read/write:
# ( in case of partial success repeat this command until errors go away. )
/sbin/fsck -fy
/sbin/mount -uw /
->You do not need to do this to change permissions. It starts up more of the OS.
sh /etc/rc
->You will probably need to press the return key once the system stops typing.
->To find out the users on the system type, use the ls command. The l is a lower case L:
ls /Users
->One of these accounts will be the administrator.