How to Reset a Lost Password on a MacBook
As a MacBook user, you should set a password and use it to log in on startup. This prevents others from making changes to your user account without your knowledge, and also prevents prying eyes from seeing sensitive data. If you forget your user password, use the system administrator's account to reset the password to something you will remember.
Reset User Password
Log out of your user account. Click on the "Apple" menu, then scroll down to "Log Out." Click the blue "Log Out" button when the confirmation window asks if you are sure you want to quit all applications.
Log in with the system administrator account and password. Choose "System Preferences" from the "Apple" menu. Select "Accounts" from the "View" menu. Click on the padlock icon. Enter the password for the administrator's account.
Click on the username whose password you want to reset, then click "Change Password." Type a new password in the "Password" text box, then type it again in the "Verify" text box. Create a hint to help you remember it next time, if desired.
Click "Change Password."
Reset Lost Administrator Password
Insert the Mac OS X Install disc that originally came with your MacBook. Select "Restart" from the "Apple" menu.
Hold down the "C" key when you hear the startup chime. Select your preferred language and click "Continue."
Click on "Utilities." Select "Reset Password." Select the Mac OS X boot drive. Select the user account you wish to change and type in the new password.