How to remove password on Mac mini

Resetting Password and/or Home Folder Permissions

What about if you forgot the pasword?

For Lion and later:

OS X Mavericks and later: How to Easily Reset the Administrator Password

Reset the user password in OS X Lion, Mountain Lion, Mavericks and Yosemite

For Snow Leopard and earlier with installer DVD

Mac OS X 10.6- If you forget your administrator password,

OS X- Changing or resetting an account password (Snow Leopard and earlier).

For Snow Leopard and earlier without installer DVD

How to reset your Mac OS X password without an installer disc | MacYourself

Reset OS X Password Without an OS X CD — Tech News and Analysis

How To Create A New Administrator Account - Hack Mac