Boot from the MacOSX set up disk that came with your computer. When it loads go to 'Utilities' and go into 'Disk Utility'. Choose the disc in the computer and go to erase (not sure if this is the exact procedure, I'm working from memory here). When choosing the type of erase I'd recommend at the very least a single pass wipe but I tend to go for the 7-pass option. After this is complete, carry on with the install choosing all the default options. I then power off once the setup is complete so that the person buying your machine will have the fresh OOTB experience and can set up the OS with their own usernames etc.