Hold option on startup or c and choose to boot from the install disk or USB.
Once loaded up go to disk utility and erase the hard disk and do a 3 pass zero out. That way it'll be harder to get data back. Once that's done, should take 3 hours or so, install Mountain Lion and once it's done just leave it. This will be exactly how it comes new.

No worries. Just make sure to do the zero out to stop anyone getting your data back.
Hold option on startup or c and choose to boot from the install disk or USB.
Once loaded up go to disk utility and erase the hard disk and do a 3 pass zero out. You can do a 7 pass zero out or even a 1 pass but i find 3 a good trade off with time and security. Once that's done, should take 3 hours or so, install Mountain Lion and once it's done just leave it. This will be exactly how it comes new.