Go to the Erase tab at the top of the window. To do this, ensure you have an appropriate internet connection for use with Internet Recovery, and then reboot with the Option-Command-R keys held to force loading to Internet Recovery. This will access Apple's servers and download the installation tools for the correct version that came with your system. In this tab, choose '1 Partition' from the drop-down menu, and then give the partition a name, set it to be 'Mac OS Extended Journaled ,' and apply the changes.
In Disk Utility, select your internal hard drive listed above any volume names contained on it, and you should see a 'Partition' tab appear. Then after selecting your language choose Disk Utility from the Utilities menu. To reinstall the original version of OS X if your Mac came with a gray installation and restoration DVD, insert this into the optical drive and reboot with the C key held down. I work at a hospital and our IS guys know nothing about Macs. From there, you can upgrade to the latest version if you choose. Select Disk Utility and erase your HD, just as instructed to do in the first section of this article. Thankfully, we no longer have to mess with DVD. How to Create a Bootable USB Drive With OS X Snow Leopard.Explaining the Mac OS X Lion Clean Install.
