As you can see in the photo to the left, Sharks forward Melker Karlsson has a nice light above him and is exposed properly, but as soon as he walked forward/past the light above him like in the photo to the right, he looks much darker and he appears underexposed. There is a pretty big difference in these two photos even though they were taken seconds apart.
Sometimes none of your lighting will be good and you will have to use whatever ambient light you have available. In that case, your best bet is to shoot with a low shutter speed, a wide aperture (f 2.8 or lower), and a higher ISO. So, while you're at home, try to photograph your subject (or furry friend!) by a lamp, overhead light, or even a flashlight to see how different your results will be.
NOTE: When shooting at a low shutter speed, try not to go under 1/100 or else you're likely to get a blurry image. The aperture is what controls how much light goes into the lens, so a wide aperture provides a larger hole to bring more light in, this would be f1.2-f4. Finally, you can set your ISO to auto to help with exposure if you need to but remember, the higher the ISO (2500 or above) the grainier your images will look.
Window Light:
Window light can be your best friend, especially for portraits! And fortunately for you, there are more windows in your home than there are in the Sharks locker room.
Different size windows will give you a range of results - bigger windows will give you more light to wrap around the subject, while smaller windows will give you a more focused beam of light with less wrap around.
Take a look at the image below of Radim Simek looking out of an AirShark window: