Welcome to Discussions - go to Applications>Utilities>Disk Utility and select Erase - it will zero out the drive so that all data is overwritten with zeros.
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You will need to boot from your install disk or from a remote disk if you want to zero the drive. You can't zero the drive you are booted from, just erase free space.
You will also need to click the Security Options button in the Erase section to choose to zero it. (Avoid the multiple pass options unless you have a good reason to choose them-they can take a very long time to complete.)