Lighting
Lighting is crucial in photography. Since photography is simply capturing radiant light, light bouncing off of things, without light there is nothing to shoot. All photos would just be a black rectangle! The trick is learning how to use that light efficiently. This link explains the different types of lighting and how to use each to your advantage. I strongly recommend reading it as it will make you a better photographer. I have compiled a basic introduction to lighting lesson. You can find the notes for it here.