If you are using 10.5 follow these steps:
Boot into single user mode (press Command-S at power on)
Type fsck -fy
Type mount -uw /
Type launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
Type dscl . -passwd /Users/username password, replacing username with the targeted user and password with the desired password.
Reboot
For 10.5 and 10.5 only do what it says where it says username type yours and for password type your new password.
If you're using OS X follow these:
Hold Apple+S when booting to enter single user mode
#sh /etc/rc
#passwd yourusername
#reboot
Now remember in OS X for password type the PW you want and type your exact username otherwise it wont rexognize a name you just created it has to be your exact one.
Hope this helps!