Were do I find a boot disk?

I recently acquired a Powerbook 540c. This is my first Apple computer, so I'm having a little trouble. Upon powering up, I am greated with that wonderful apple chord and a floppy with a blinking question mark on it. I assume that this means the computer wants a boot disk. If so, where can I download one? How do I set up the computer to boot without it? And if not, what does it mean and how do I fix it? I don't know if this helps, but holding down apple and backspace produces a nasty sound, a black screen with a dead mac, and the code 00000E 00000F or something like that.

Thank you for your help,
TLAK