Currently Being ModeratedMar 6, 2013 6:30 AM (in response to la_iwana)
Very simple Pablo!
First backup your system completely to an external HD using either Time Machine or create a bootable clone.
Next restart in the Recovery Partition built into your system. Google it if you don't know how to restart into the recovery partition.
Start Disk Utility and reformat the interal HD then exit DU and select Install Mountain Lion and last if you want to restore from your backup then choose restore from Time Machine.