When you get the disk, boot with the disk in the drive while holding down the "option" key. That should bring up a boot choices screen, use the arrow key to select the install disk. When that boots, go to utilities on the top menu bar and select "disk utility". Erase the hard drive, then close disk utility and continue the installation.
Then you should run the updates, in the upper left hand "apple" logo go to "software update" and run that a couple of times until no more new updates are available.