Are you sure you want to format? That will destroy everything on the hard drive.
If you're sure that you want to, put the XP cd in your cd drive and reboot - no need for a floppy.
If it won't boot to the CD-ROM forst, you will need to change the boot order. This must be done by entering the computer's BIOS. This is done by pressing a specific key on boot-up. It varies from computer to computer, but the most common are DEl, F2, or F10. You should see a brief message on bootup telling you what key is appropriate for your computer.
When you can get into your BIOS, look through the options until you find the section that deals with boot order (sequence, etc.). Change it to reflect that you want the machine to boot from CD-ROM first.
OK, then you should be able to boot from the CD.
Follow the instructions to do a clean/full install and pick the appropriate hard drive when prompted. Click next until done.
There's no going back once you start a format (for the most part).
Good luck!