Power Mac G4 Cube "help"

My dad got a Power Mac G4 Cube almost free from a friend. I have no idea how to use a mac, so I got some problems. First when I started the mac with the mac monitor nothing comes up on the monitor, I have no idea why. When I start the mac the power light on the monitor goes on but the monitor is still black. So next I plugged in my PC monitor instead, when I did that something shows on the monitor, the desktop is grey with a folder with the mac logo and changing between the logo and "?". I guess it means that there is no OS on it? And what about the mac monitor is it destoyed?

#48385

I guess there is something wrong with the Mac monitor.

And the blinking icon indicates the absence of an OS..

#48393