Put a boot disk in the disk drive and hold C on the keyboard while starting.
This will force the computer to boot off the CD-ROM drive if the disk in there is bootable.
If you can't get the drive open, hold down the mouse button while powering on and it should open.
B&W G3s shipped with OS 8.5.5. Unless you have a specific reason for needing OS 8.x(if you do, I suggest running the 8.5.5 install and then upgrading to 8.6), I'd suggest going straight to OS 9. I prefer to use a 9.2.1 retail disk and upgrade to 9.2.2(Apple never released a 9.2.2 retail disk-the closest you can get is the 9.2.2 restore disk that shipped with Quicksilvers and a few other G4s, which works on a lot of computers but I don't think a B&W G3). There's also the Mac OS 9 Lives 9.2.2 universal install, which is probably the easiest route.
Forum rules prohibit posting links to downloads of any of this software, but disk images can be found on the internet from reputable sites.