Currently Being ModeratedFeb 3, 2013 4:42 AM (in response to antonisfromathens)
As you are going to erase the hard drive, you must make a backup of your data. Then, follow these steps:
1. Press Command and R keys while your Mac is starting to start into OS X Recovery, and open Disk Utility.
2. Select the hard drive at the top of the sidebar, go to Erase tab and erase the disk.
3. Close Disk Utility and reinstall OS X.
After the second step, the hard drive is erased. However, if you don't reinstall OS X, you won't be able to start the Mac after restarting
Currently Being ModeratedFeb 3, 2013 4:54 AM (in response to antonisfromathens)
You restart your Mac when you at the same time hold down CMD and R buttons on your keyboard. Make sure your machine is connected to the internet at the same time.
Now select Disk Utility and click continue. Then select your startup disk from the list on the left and click the erase-tab.
From the Format pop-up menu choose Mac OS Extended (Journaled), enter a name for your disk and click "Erase"
When operation is finished choose Disk utility > Quit Disk utility.
If you want to reinstall OS X again, then select this and follow further instructions