According to this website. All you have to do is this:
Click the Apple icon in the menu bar (top-left corner).
Select System Preferences from the drop-down menu.
Select 'Erase All Contents and Settings.'
Enter your password, select 'OK.'
Select the 'Erase All Content & Settings' button.
From my systems preferences I can not see "Erase All Content and Settings"? Is it just me?