1. Insert your Mac OS X CD-ROM disc or Restore DVD disc, then restart the computer while holding the C key until you see the spinning gear.
2. Once started up from CD or DVD, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from disc to access Disk Utility.
3. Choose the entire disk (top icon).
4. Choose erase tab.
There you can enter the volume format, name and click erase. The disk is now reformatted.