How to do fresh install of Mountain Lion? My Mac is very slow.  

My Mac is running OS X Mountain Lion after an upgrade from Lion.  

How can I do a fresh install the OSX Mountain Lion, with a clean HDD (all of my documents deleted), so it feels like a new MacBook?
Back up your data.
Boot into the Recovery System by holding ⌘+R during early boot.
Select Disk Utility and format your drive where OS X and your files were stored on.
Quit Disk Utility and select "Install Mountain Lion".