If you would provide more information about your computer...
including Bios and version, it would be easier to help you.
Also, it would help if you would tell us why you are now doing what you are doing.
(What happened to make this necessary?)
Different bios makers (and versions) require slightly different approaches,
If you need detailed direction... we need to know what we are working with.
Otherwise,
Going into the bios, make sure you write down ANYTHING that you change.
(this will enable you to change it back).
The ONLY thing you should change is the boot sequence,
moving cd to the top of the list, and your hard-drive as second.
This should allow you to boot from cd.