After having looked at the rules of composition let's summarize the elements of composition.
Abstract Design: learn to see shapes, color, masses rather than "objects" such as a tree, a house, a road, a flower...Look in a mirror, squint or or look at things upside down to help you see these "blocks".
Balance: the "blocks" should be more or less equally balanced in all parts of the painting (hint: divide it in 4 equal zones with a cross) in term of size, color, temperture, contrast.
Tension. Too much balance is boring, so use asymmetry vs symmetry for instance (today's trend among artists)
Focal point: The point where the viewers eye is naturally going and rest there before exploring aother parts.
Rules of thirds: a way to know where to place the horizon, the focal point...
Also mind the negative vs negative spaces. Looking at the negative spaces (shapes between the forms) can give you a new way of looking at your painting..
Here is phantastic website where you can explore the subject very well.