How do I reset a log in password when I can't remember the old password

I have the same question, same problem, can anyone help ?

Administrator Password?

Reset Password

Lion/Mountain Lion

Follow the instructions in the beige and green boxes.

http://pondini.org/OSX/Password.html

Note

You should now be able to log in with the new password, but you won't be able to unlock the Keychain. If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it. You’ll need to reset your keychain in the preferences of the Keychain Access application.

http://support.apple.com/kb/TS1544