What the others said - to summarize, it is "necessary" to study not only the theory behind things such as anatomy(as in studying the layers below the skin as well), perspective(such as point perspective), composition(like the golden rule), and color(color wheel, complementary colors, ect.).
You should also draw from life, because that is what will tie the theory together with practice and make learning more effective.
Good points above, but the basics also include composition (the position of the objects on your canvas), positive and negative space (is your work balanced or too crowded/emtpy looking), and visual flow (how the eye travels through your work, ie, the eye should be drawn to the most important aspects first, then move on to the second most important, and so on).
Well, there are some thing every artists has to know about and that is basicly what you should learn during art-class in school. To specify they are: Anatomy: How humans are built, and their propotions Form: Knowing that every object can be broken down into three basic shapes: The sphere, the rectangle and the triangel. Knowing these will give you an unlimited ability to draw whatever you want. Shading: Knowing how shadow falls on objects and people, the logic behind highlights and so on. And coloring: Knowing one or more way to colour your art is just as important.
Those are according to me; the basics. But you should allways be willing to test different things like scoulpting.