To boot from the CD-ROM, you have to hold down "C" right after you hear the startup chime. If you hold down Command-S, that will put you into Single User Mode, which is where you go when you are having problems logging in or booting into OS X.
If the "C" option doesn't work, hold down Command-Option-Shift-Delete. This will force the Mac to check for all other bootable devices before going to the hard drive.
Also, holding down the "Option" key while booting will present you with a GUI where you can select from the available boot devices, if you already have OS 9 installed.
If your computer boots into OS X just fine, simply open the System Preferences, select "Startup Disk" then select the OS 9 folder (it may take a moment or two for all options to appear as it scans the disk). Then, restart.