Shut off your Mac. Then hit the power button to turn it on and as soon as you hear the chime sound press the command and r keys at the same time. That will force it to boot to the recovery screen.
Then... Do a command-r boot and this will take you to a recovery screen. Launch Terminal from the Utilities menu then type "resetpassword" (without the quotes) and hit enter. Now pick your account and enter a new password.