As you are running Snow Leopard, you need the Mac OS X DVD that came with your computer. If you haven't got them, call Apple to get replacement DVDs > http://support.apple.com/kb/HE57
Then, follow these steps:
1. Insert the Mac OS X DVD and press C key while your Mac is starting.
2. When the disc has started, go to Utilities menu > Disk Utility, and select Macintosh HD on the sidebar.
3. Go to Erase tab and erase the disk.
4. Close Disk Utility and reinstall Mac OS X