Resetting Your OS X Password
If you have misplaced, forgotten, or do not know any of the administrative passwords on your own OS X computer, resetting it is quite simple.
To do this, you will need an OS X Installation CD that will start up your computer. It is best to use the version of OS X that most closely matches the version you are currently running. One of these comes with all Macintoshes that shipped with OS X installed; if yours did not ship with OS X, you most likely bought a copy of Jaguar (OS X 10.2; this comes in a white box) or Panther (OS X10.3; a black box).
– Insert the Mac OS X Installer disk.
– When you hear the startup tone, hold down the C key until you see the spinning gear.
– When the Installer appears, choose “Reset Password” from the “Installer” (for 10.3) menu. Note that for 10.4 the menu is called “Utilities.”
– Follow the onscreen instructions to change the password.
– Quit the Installer and restart your computer while holding down the mouse button (this will eject the CD and prevent you from going back into the installer again).
Apple’s technote on resetting the administrative password
More about passwords in OS X
How to change a user password (when you do know an administrative password)