1) Make sure you are in the finder application.

2) In the toolbar at the very top of the screen, click "Go".

3) From the drop down menu, click "Utilities".

4) In the Utilities folder, find "Disk Utility", and double-click on it.

TO OPEN DISK UTILITY, click here ---> Open Disk Utility.

5) In the menu to the left of the window, click on the hard drive that you want to securely erase all information.

6) Locate the tab bar. (First Aid, Erase, Partition, RAID, Restore)

7) Click on Erase.

8) Follow the list of instructions in the window. (Instructions may vary, depending on what model computer you have)

For more information, visithttp://docs.info.apple.com/article.html?path=Mac/10.6/en/8493.html