Can't really recommend any specific guides but all cameras operate in essentially same ways. Some have more control than others, but the G15 should have plenty enough I should think.
Best thing to do is learn the basics of photography. Aperture, Shutter speeds, composition, light, white balance, etc.
Aperture and Shutter speed are paramount to operating a camera. These are two things you'll always be thinking about. To make things a little easier, most photographers, unless shooting very specific things that require otherwise, will shoot in semi-automatic modes, like Aperture Priority, or Shutter Priority. Aperture priority I think most people find useful...shutter priority for when you need strict control over shutter speeds.
Look these things up and it will start making sense. Then look at the manual to figure out where these controls are.