How can I install Debian on a PowerBook G3 with Mac OS 9.2?
According to this how to article:

You'll need a working OS X partition and you'll need to reboot with Option + Command + O + F. It is a bit complicated so follow the link for more info.

I personally would go with Ubuntu on your Powerbook. Follow this guide for info on how to install. Although the guide is for a Power Mac G4, it should work with your Powerbook.