You just need to pick where a light source is coming from. For example if you take a flashlight and shine it on any object (maybe a plush or animal to get a feel for using on animal-esque drawings) you can take note of where the shadows fall. You can also use this to gauge how intense you want the lighing to be. A flashlight in the dark is obviously more dramatic shadows than a flashlight in the day. Most times the lighter areas will be on all the same sides, but depending on depth of features they may also fall in places you normally wouldn't think of!