Select “Disk Utility” and then click Continue.
Select your startup disk from the list on the left, and then click the Erase tab.
From the Format pop-up menu, select Mac OS Extended (Journaled), type a name for your disk, and then click Erase.
After the disk has been erased, choose Disk Utility > Quit Disk Utility.
If you’re not connected to the Internet, choose a network from the Wi-Fi menu in the right corner of the menu bar (in the top-right corner of the screen).
Select Reinstall Mac OS X, click Continue