Reformatting an iMac

Hi all,

Super basic question:
I'm a Mac noob; I just bought an iMac off a friend of mine, but he didn't bother to reformat before giving it to me. Although he gave me the installation disk, he didn't have any restore CDs, and that's the only way mentioned in the "Emergency Handbook" to wipe the hard drive. I don't know my way around a Mac at all -- How do I reformat the silly thing?

Thanks!

do you REALLY need to, or just wish to prune off stuff you don't want?
Click on the desktop empty space(not an icon). The upper left icon in the
menu bar is the Apple Icon; click and slide down to About This Mac.

What version of the OS is running?