Silly question first, are you booting the PC with the CD already in the drive, as you must.....if you are, someone has changed the boot order in the bios. How to put that right? Hit the correct key as your PC boots.
It varies depending upon what BIOS your computer has. Look for a message on the screen as soon as the computer boots. Common keys are F10, F11, Del, or ESC. You will have to check the screen or the manual that came with your PC.
Once in the bios, the boot order is usually on the advanced page. You want to bring the Cd/DVD drive to the top of the order, and HD second. Save the new bios order. Reboot with the CD in the drive already and it will boot off it. Follow on from the above links. Watch your step, messing with the bios can cause trouble, picking an incorrect option from the install CD can loose your entire drive.
I must be blunt. You are solely responsible for any bad results from our attempts to help you, and as you clearly have no idea what you are doing, I suggest evening classes would be a good step.