OK, Relax. Are you booted from the Recovery HD partition? If not, then you need to first boot from the Recovery HD before you can erase the Macintosh HD. If you are, then you neeed to do the following:
Select Terminal from the Utilities menu. At the prompt enter:
diskutil list
Press RETURN. From the listing look on the left column for your disk identifier. If will look like /dev/diskn/ where "n" is an integer starting at 0. You should also see the name of the disk such as "Macintosh HD." Note down the disk identifier. For a single drive system this will probably be /dev/disk0.
Now enter:
diskutil unmount force /dev/disk0
Press RETURN. This should unmount the drive so you can now do this:
Install or Reinstall Mavericks, Lion/Mountain Lion from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Erase the hard drive:
1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your startup volume (usually Macintosh HD) from the
left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on
the Security button and set the ZeroData option to one-pass. Click on
the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.
Reinstall Lion/Mountain Lion, Mavericks: Select Reinstall Lion/Mountain Lion, Mavericks and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.
