You can't boot into anything, because you literally have nothing on your hard drive. Don't bother with OpenFirmware. The best luck you will have with operating systems is either 10.4 Tiger (
here), which is a very good deal, or 10.5 Leopard (
here). If you want speed, go with 10.4 Tiger. If you want more compatibility and more modern software, go with 10.5 Leopard. There is always Linux, however, if you don't feel like spending any money. I would suggest going with something like Lubuntu 12.04, or a version of Ubuntu. You could also try MintPPC, Debian, or YellowDog. But Linux won't give you as much of a 'bang' as OS X. Don't expect it to be the best of the best, because, for more money, OS X could do a better job, but Linux is your friend if you don't want to spend money.
Good luck!
-jrsx