If you have an iMac with Mac OS X 10.5 Leopard or Mac OS X 10.4 Tiger, look for guides on "Erase and Install Mac OS X" via a www search engine.
PS: Your signature lists the "iPhone" as "Iphone" and says "32gb" instead of "32 GB". Just a minor annoyance on my part, nothing earth shattering, especially after the world has already ended anyway.