Q: delete all contents on macbook

Hello spiritosa_3,

The process to perform what is referred to as an 'Erase and Install' (erasing the contents of the computer and installing only the operating system) differs slightly depending on the version of the Mac OS you're running. The steps are the same for Lion and Mountain Lion, however.

Mac OS X v10.6 Snow Leopard: How to Erase and Install

http://support.apple.com/kb/HT3910

Use these steps to restore to the factory settings and the software that originally came with your computer by using the installation DVDs that shipped with it.

OS X Mountain Lion: Erase and reinstall OS X

http://support.apple.com/kb/PH11273

You can erase your Mac and then reinstall OS X using the built-in recovery disk. Before you erase and reinstall, back up your essential files.

Cheers,

Allen

Glad I could assist!