Learning guide: composition

Composition, like your colour choices, is often a matter of intuition. Your painting is a window to another world with its own rules and stories. There are such things as visual "weights", or the balance of all the elements in your painting, visual rhythms that lead the eye around the picture, points on which we want the viewer to focus, contrasts and unities, mysterious places that are meant to rest the eye where the detail is smudged and lost, and places to excite the viewer with full detail and clarity. How do we obtain an intuition for composition? It's simple: look at a lot of paintings. Spend at least 10 minutes a day looking at great paintings, view as many as possible or focus on your favourites. Start a folder on your computer, call it "My favourite paintings", fill it with 50 paintings you admire and make sure to look through it often.
Go here to find paintings to save to your folder:
https://renata.forumotion.com/t13-old-masters

If you want a more formal approach to learning composition, check out
The Painter's Secret Geometry: A Study of Composition in Art
Charles Bouleau

Useful videos
https://www.youtube.com/watch?v=fszXLO6fURY

https://www.youtube.com/watch?v=SF7nvgw2pPk

https://www.youtube.com/watch?v=qxGwNVNrB64

https://www.youtube.com/watch?v=RHXEzU5akLo

https://www.youtube.com/watch?v=86eC3KXB1oA

https://www.youtube.com/watch?v=-l_Pn7G0hK0

Just remember whenever you learn formal rules for composition: you can break them all once you know what they are, and still have a successful composition.
A good intuition beats great rules almost always.
So keep looking at paintings, keep learning, and good luck everyone!

A great way to improve your composition is to practice small thumbnail sketches. If you figure out your painting's composition while it's small, it serves as a map for the final piece.

Great video
https://www.youtube.com/watch?v=XDqY8IFcb5Q
You can of course find shorter guides to making thumbnails, but this one is very detailed for anybody struggling with getting the ideas out into their sketchbook.