Composition is an abstract language that can be manipulated to achieve artistic goals. Put in more plain language, it's the act of thinking from above in abstract shapes that will, even before anything is drawn, express the idea the artist is trying to communicate.
Triangles are a very good abstract shape to work with, as the brain focus more on the angles as focal points than the center of its shape like when using a square. It makes the eyes move into the frame and can convery different feelings.
Outside of triangles, composition can use squares, rectangles and parallelepiped. Depending on their regularity and placement they can express opposite feelings.
Composition can be used in a macro-sense to give a broader intention, but also in a micro-sense, like in architecture or for shapes of an object. But the point is not to put golden ratios everywhere or calculate things.
Points to remember:
- Don't start drawing or painting right away
- Think about the intention of the drawing/painting
- Use abstract shapes to support the intention
- Don't overdo it