just download the software from the app store, then create a usb boot drive. Alternatively, if you have the recovery partition (which I think was included in a later software, like 10,7 or 10.8), reboot the computer, hold commanf and R until you hear the sound, then you can release. when it gives you the options, choose disk utility, then format the primary hard drive, then install the OS.
Also, just remembered that if you don't have the recovery partition, you should have the included recovery disks. No matter what you do, just make sure you back up anything you want to save, and having fast internet helps if you go with the recovery partition route.