This article explains how to perform a clean install of OS X Mountain Lion on your Mac's startup drive. It includes information about the decisions you make when performing a clean install.
How to Mount OS X Lion to a Flash Drive/Hard Drive and Clean Install Mac OS X Lion
If you want to install Mountain Lion to your Mac’s startup disk, you’ll need to first restart your Mac from a bootable copy of the installer, because your startup disk will be erased before installation. Do this using Disk Utility, which is included with the installer.
Insert or connect the bootable media to your Mac and reboot your Mac while holding down the Option key. This will bring up your Mac’s built-in startup manager, where you will select the device you want to boot from. Use the arrow keys to select the Mountain Lion bootable installer you created earlier, and then press the Enter key to begin the boot process. The Mac OS X Utilities window will appear as if you are booting from the Recovery HD partition. There is no Recovery HD partition available yet because you have not installed the operating system, so you have created your own bootable media.
Select Disk Utility from the list of options, and then select Continue.
