how to format my hard disk

Restart with the Command and R keys held down and do it from the Disk Utility on the recovery partition.

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You do this either with a Mountain Lion install disc or drive, which you have to make yourself, or from the Recovery System. For the Recovery System, you reboot while holding down command-R. This boots from a (hidden) recovery partition that is on your hard drive; then you access Disk Utility and do your formatting that way. The Recovery system can do a new install of Mountain Lion from the internet from apple once you have reformatted your drive. Or you can make a ML installer disk from the installer files that you download for future reference.

Here's a page about Recovery mode: OS X: About OS X Recovery

and here's how to make your own ML installer disk: How to Burn OS X Mountain Lion to a DVD or USB Flash Drive..

BTW, you could have figured out all this yourself with a google search.