Boot it from the snow leopard cd (right after it turns on with the CD in, press and hold down "c" and it will boot from the cd).

Once it comes up with the language or whatever, there should be a "Utilities" drop-down at the top of the screen. In that drop down select "disk utility." There you can select the hard drive and "erase" it and you should be good to go. You can then install a fresh copy of Snow Leopard.

And don't worry, Mac stuff is usually very simple and straightforward, and if you don't know how to do something that you know how to do on a PC, just google it, and you should be good.