For the most part, there are 2 very common causes of not being able to boot from a CD.
The first is the most common:
The CD you are trying to boot from is NOT bootable! You can use Google to find out how to make a CD bootable, I am not going to cover that here.
The second cause is easier to fix:
You simply have to change the Boot Order in your laptop's BIOS. To access the BIOS, hit the F2 key right when you turn your laptop on (while the Dell symbol is still being displayed). The menu that loads is called the BIOS. Press and hold the ALT key, and press the P key to move through the menu pages. I believe it is the second page where you will see the Boot Order settings. Simply change the first boot device to cd-rom drive and then hit the ESC key and choose YES if prompted to save. Hit CTRL+ALT+DEL to restart your computer and try to boot from the CD. I highly recommend that you set the first boot device to your hard drive so that your laptop starts up faster.