It sounds as though you may have more than one problem. To change the boot sequence, you have to go into the BIOS and change the 1st boot device to your CD-ROM. You haven't provided enough information about your computer or operating system but when you boot up you should see how to get to the "setup" by pressing a certain key. Give me more details and we'll see what we can come up with.

Depending on the age of your computer, you might want to consider installing a new floppy drive.

Hope this helps