When you boot to that installer DVD, you'll go through a couple of screens, such as selecting the language for the install, and agreeing to the license. You will then see menus at the top of the screen. Choose Disk Utility, from the Utilities menu.
Select your hard drive from the list in Disk Utility, then click the Erase tab.
Name the partition, if you like.
Make sure that partitions is set for a Mac OS Extended (journaled), then click the Erase button.
When that Erase completes, your hard drive is wiped....
Quit Disk Utility, and continue on with the install.
You said that you have an older iMac.
All of these instructions are useless if you have an older PPC iMac (like a G4, or G5 processor)
Snow Leopard will not boot or run on those, but requires a Mac with at least a core duo (intel) processor. You might want to make sure about that first, eh?
