Please provide laptop details such as make model etc. for specific diagnosis.
But generally:
1-Press the 'Delete' or 'F8' or 'F12' (different for all laptops) to go into the BIOS while booting. (When you start normally, you will see a screen with the manufacturer name flash very quickly. This will have some text somewhere saying 'to enter bios press...' etc. It is that key whichever it prompts there.)
2- Go to boot order or boot option in BIOS
3- Select CD drive to boot first.