Normally you don't need to erase your disk to reinstall OS X. The OS X Installer is designed to allow you to perform an install in place if you are troubleshooting. If you need to erase your disk because you are transferring your computer to a new owner, you can use Disk Utility to erase the disk before reinstalling. 

Important: Before you erase and reinstall, back up your essential files. If you’re installing on a notebook computer, make sure your power adapter is connected and plugged in.

Use these steps if you need to erase a disk before installing OS X:

1. Before you begin, you need to be connected to the Internet.
2. Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts.
3. Select “Disk Utility,” and then click Continue.
4. Select your startup disk from the list on the left, and then click the Erase tab.
5. From the Format pop-up menu, select Mac OS Extended (Journaled), type a name for your disk, and then click Erase.
6. After the disk has been erased, choose Disk Utility > Quit Disk Utility.
7. 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).
8. Select Reinstall Mac OS X, click Continue, and then follow the instructions.