This basically the most important aspect of taking great photos. Sometimes you want more light, and other times you want less light. But too much or too little light can make the subject washed out or too dark to even see it well. You also don't want direct light as it can make subjects looked washed out. (more on that later)
Lets talk about times you want more light. Lets say an indoor family party or reunion. You want more light to make it seem happier and we all know how kids don't stand still. More light will make it easier to catch a moment without blurring.
The obvious answer to this is to use the flash, but that often makes the subject look washed out and unnatural because of the direct bright light. There are several ways to combat this, like a flash thats not attached to the camera
, or a flash diffuser.
A better way if you're lazy is to open windows, and turn on lights. I find that flourescent lights work best because they provide bright white lights, but any lightbulb will work. You can also use work lights depending on the situations but the easiest way is to GO OUTSIDE!
Now times you don't want light, like when you're photographing a baby sleeping you'll want it to seem quiet and peaceful. I find that if its in half light, light enough that you can tell what it is when the picture is taken but dark enough that it seems dark. Think about twilight as the sun sets.
Close windows or just cover them with curtains so the light can get through but not too much. Or leave the door open with a light in another room.