Start up from your system installation disk by inserting disk and stating holding down the "C" key. Go to the second screen pull down menu - Utilities -> Disk Utility and format the drive. Now go back and install a system.
Make sure you get a backup software. There are tons on internet, choose what suits your demands. I personally use www.safecopybackup.com. If want to give a try, they have 3GB free trial. You'll like it.