If you have OS9 on a seperate partition then you can also hold down option when the machine starts to choose a startup disk, which is useful if you only need to do it occasionally. Remember that new machines (last bunch of G4s and PowerBooks from about March-ish) won't boot into OS9 anymore.
Depends on the machine you have. Newer machins won't boot into OS 9. If it's not one of those, try hlding the Option key down while booting. You'll get a screen giving you a choice of what system to boot into, unless it's a good bitolder.
Next post, please give more information, Model, Ram, OS, etc.