I want to install 10.3 How would I boot from a CD?

Two ways that I know of.

Pop the CD in while in OSX, a finder window opens, and click on "Install Panther" (or similar) which will then restart your machine, for you.

Or, boot your Mac up and hold down the C key after you hear the chime, let go after you reach the white loading screen.

or you can just restart with the CD in the drive and the Mac boots from the CD.

You can also go to system Preferences and click start up drive with the OS X cd in and you can choose to boot from it