It's very important to erase your HD securely as simply deleting the files and placing them in trash will not necessarily do this.
Fortunately, MAC OSX comes with a very handy applications called Disk Utility:
Applications / Utilities
After you have backed up all the important files you wish to keep insert the Install Disk that came with your Mac into the Superdrive.
Reboot your Mac and hold down C during this process.
Your Mac will boot from the disk.
Choose Disk Utility from the Installer Menu.
Select Erase and click on 'Security Options...'
35 pass is the most secure (but will take a long long time). I usually go for 7 pass personally.
Then you can install a clean OSX using the Install Disk.