My computer turns on but doesn't start up

Ok, I have a problem with my iBook G4 12 inches, Mac Os X Leopard 10.5.8, 1.33 GHZ and 1.5 GB of Memory.

The problem is that when I turn on my iBook it turns on and it shows the apple logo and the charging icon after that, it shows three blue screens (all this is normal), but the problem is that after the last blue screen it doesn't show my desktop because it stays in the third blue screen. It only show the pointer with the spinnig wheel and it doesn't pass to the start up process and I can't do anything. I try to start up with the command key pressed to choose another start up volume but if I try it the screen doesn't turn on although I can hear the start up chime and the fan. And also the gray screen that appears after press the turn on/off button has lines from up to down in all the screen and also the apple logo has lines from up to down inside it.

Do you understand me?

Please help