The ISO settings on most digital cameras range from 100 ISO to 3200 ISO and higher. On a bright sunny day, you can set your camera's ISO to around 100 since so much light is available. Inside, or when it is dark outside/or shade, you can set your camera's ISO to around 800 to 1600. If your image is still underexposed, then you can try using a flash or changing your shutter speed and aperture.
Here is an example shot outside in the shade. You can see the difference when you change your ISO.
|ISO was 100|