In order to change the administrator, known in Unix as "root", password you will need a copy of Mac OS X Install disk. If you don't have the original disk, buy another one or call Apple (1-800-SOS-APPL) and ask for a replacement boot disk.

Once you have an install disk, double click on the Install Mac OS icon and you'll get a window with a restart button. Click on it, and your Mac will restart and go to the installation process.

You don't need to install the OS again! Click through windows until you get the regular set of choices on the menu bar. One of those menus is Utilities and one of the choices on that menu is Reset Password.

Change your password and choose Quit from the Installer.

Next time you start your computer you know your admin password.

Note: your admin password is only of limited security because anyone with an OS install disk can restart and reset your root password without knowing your existing password.

How to reset Mac OS X Admin Root password by using sudo command