Click on Utilities menu > Reset Password [Note that when you’ve reset the Administrator’s password you haven’t reset the login keychain password—that password remains as it was when it was originally set. Once you login the individual passwords within the keychain will have to be reentered].
Assign Firmware Password:
Bootup holding Command + R keys
Click on Utilities menu > Firmware Password Utility … Change … Require Password to Change Firmware Settings [When a user starts up your Mac with an installer disc in the media drive and presses the C key to boot from the disc, the Mac will ignore that key press and move to the login screen. If he is really savvy and presses the Option key to move to the screen where you choose a device to boot from, a padlock icon appears with a text field below. If said kid doesn’t enter the correct firmware password in that text field, he or she won’t be able to boot from the disc]
Use recovery mode in order to reset your password
Turn off the Mac, then press and hold the Command and R keys on the keyboard and then press the power button while still holding these keys.
Continue to hold the keys until a progress bar below the Apple logo is seen. Your Mac is now in Recovery mode.
In the Recovery mode menu, select Utilities then Terminal.
Type "resetpassword" in the command line as one word (no quotes) and press enter.
Close the Terminal window (command line window) and notice the Reset Password Tool.
In order to reset your password, you have to reset every users. Write down every accounts password that is changed.
Once done, restart the Mac and log in with the new password.