1) Click on the Apple in the top left-hand corner of your screen.
2) Scroll to System Preferences
3) Under System, choose Accounts.
4) Click on Change Password...
5) Type in your old password (the first time you change the password, it will be the default password which came with your computer), then type in your new password (I recommend you create a password consisting of letters and numbers) in both the New Password field and the Verify field.
6) Click on Change Password; your password is now changed.