Well I don't know anything about Debian, but I am running Gentoo on my G3 iBook (2003) and it is an awesome distro. Best of all, there massive/comprehensive install manual WILL tell you everything you need to know
http://www.gentoo.org/doc/en/handbook/handbook-ppc.xml. That, and this page:
http://www.desertsol.com/~kevin/ppc/ where a knowledgable Gentoo/iBook user describes his install process and provides config files for kernel, X, etc. etc.
If you are sold on Debian then by all means... I just figured I'd throw Gentoo out there because of it's most excellent documentation.
Here is the "about" page. I have a friend who is a complete newbie and he just completed his Gentoo install
If you really just want to get started and not spend a lot of time on installs, Mandrake makes a PPC version, and there is also Yellow Dog. I tried Yellow Dog for a few days and everything simply worked right off the bat. Pretty cool for a new user.
As I said I don't know much about Debian, just thought I'd throw some other PPC options out there for ya!