I'm Selling my macbook, How do i erase all my data please

Buy a USB external drive and "clone" your drive to it using SuperDuper. Then follow these instructions to erase and install a new system.

http://support.apple.com/kb/HT3263

EDIT: I thought it was a MacBook Air. Was in the wrong forum.

Dave M.
MacOSG Founder/Ambassador  An Apple User Group  iTunes: MacOSG Podcast
Creator of 'Mac611 - Mobile Mac Support' (designed exclusively for an iPhone/iPod touch)

Hey there,
Perhaps this older thread will answer your question. Hope this helps.

http://discussions.apple.com/thread.jspa?threadID=726962

B-rock

Insert Install Disc 1 and boot the computer while holding down the C key. The OS X installer should load and you should choose the "Erase and Install" option.

~Lyssa

Assuming you still have the discs (OSX and Applications) that came with the machine, you can use them to restore the HDD to "out of the box" state.

Before you do that, I'd recommend you download and run a utility called DBAN. This will completely wipe any trace of old data from the drive. Believe it or not, even if you format & restore the factory data to a drive, it is still possible to recover the old data from it (though not easy). DBAN overwrites every bit on the drive multiple times to completely scramble the data that existed on it. DBAN can be run from a USB flash drive or burned to a "live" CD (meaning the machine will boot from the CD instead of the HDD).

When 'erase and install', select the 'security options' and use 'write data with zeros, 7-Pass or 35-Pass'. That will erase your private data if the new owner wants to 'check what was there before' he bought it.