Lastly, if your images are still turning out slightly too dark or bright, you can use exposure compensation to correct for that error.
To sum everything up, I recommend shooting in aperture priority mode. In low light, set your aperture as wide as possible, and bump up your ISO until your shutter speeds are fast enough for your subject (meaning at least 1/250 for a lot of wildlife). In brighter situations, you have the luxury of lowering the ISO and getting better image quality. Lastly, if your images are still turning out slightly too dark or bright, you can use exposure compensation to correct for that error.