When planning your pictures, decide what mood you want to convey. That will impact your choice of lighting.
In this picture, I wanted a serious look. This, though, was partly dictated by the available light. We were staying in a hotel, and the balcony door was north facing with the forest and mountain reducing the amount of light reaching our room. The exposure was set according to the light falling on the boy’s face so that the background, which was medium brown, was thrown into shadow. Honestly, the room wasn't as dark as it looks here.
Assignment: Study pictures in magazines or online and see how the lighting affects the mood of the picture.