You might find this link useful:
http/support.apple.com/kb/PH14243
"Erase your Mac, then reinstall OS X using the built-in recovery disk. Before you erase, back up your essential files. If you’re installing on a portable computer, make sure your power adapter is plugged in.
Important: To reinstall OS X, you must be connected to the Internet.
1. Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys.
2. If you’re not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
3. Select Disk Utility, then click Continue.
4. Select your startup disk on the left, then click the Erase tab.
5. Choose Mac OS Extended (Journaled) from the Format menu, enter a name, then click Erase.
6. After the disk is erased, choose Disk Utility > Quit Disk Utility.
7. Select Reinstall OS X, click Continue, then follow the onscreen instructions."