If its a dell computer, you should have a lodaing screen with the word DELL written in the background and a loading bar at the bottem. At the top right cornet it should say 'F12 - Boot Menu'
Press F12 before the loading bar fills up. You will be directed to a place where you choose where to boot from.
Use the arrow keys to navigate to the CD Drive and press enter and your computer will boot with the CD.