Depending on your BIOS, most of time when you are turning on the computer and see the company logo, you should rapidly hit F2 or DEL key until you get into the BIOS screen. When you are there, use the arrow keys and navigate to the last page to the right. You will see that it says boot or power options. You should be able to see something that says boot order. Just change the list using the + or - keys to get the CD first, then HD second. If you have a floppy drive, you will want it first, then CD, then HD. Then you want to save changes and exit. It will reboot and if the CD drive is working properly and the CD is bootable, it should work.
If you have a very old computer or an HP or Dell, you might need to hit F1 or F11 to get into the BIOS. Also some computers you can tap F8 when you see the first boot logo and get a boot order popup menu and then you just select the CD or HD you want to boot from and hit Enter.
As Kinth stated, if the board is older... perhaps before 2004, it might not allow you to boot from CD. Especially if it is a Dell, Gateway or HP with a locked BIOS. If the computer is made before 2000, then it is very unlikely it will boot the CD first too.