What OS are you using? If you're on OS 9 or older, you can use the Drive Setup utility. It should be located somewhere in your utilities folder. What you would need to do is boot up using a startup disk (such as the OS 9 installation CD), launch Drive Setup, check "zero all data" and "low level format", and let it do the job. If you need more info, have a look
here.
If you're using OS X, follow this guide:
From here
1. Insert the Mac OS X CD.
2. Restart the computer.
3. Immediately after the startup sound, press and hold the "C" key to start up from CD.
4. When the Installer screen appears, do not click Continue. Instead, choose Installer > Open Disk Utilities.
5. Select the hard drive to erase.
6. Click the Erase tab.
7. Select the volume format from the Volume Format pop-up menu.
8. Click Options.
9. Select the checkbox for "Zero all data".
10. Click OK.
11. Click Erase.