Pretty simple really:
1. Once the computer is on, insert the CD. A window will open with an icon to start the installer. Double click that, enter your password, and the installer will restart the computer and start from the CD.
2. You could also just hold down the 'c' key once you start the computer if the CD is in the drive.
From the installer, if you have nothing on the machine that needs to be saved, you can launch Disk Utility from the menu (not sure where exactly) and wipe the drive clean to do a fresh install of the system. Once that is done, go back to the installer and start installing. Hope that helps
