Other than what’s already been mentioned, there are other aspects of composition that you must know, and some of them are issues that even advanced artists run into.
One of the most common issues I see in my students’ composition is how flat they present their scenes, as if every scene they depict are set up like a theater stage play with the actors facing the audience, and the viewer is sitting in the middle of the audience looking at a stage in front of them. There’s no sense of spatial dynamics or any of the more modern cinematic sensibility where the viewer is more like a camera that can placed anywhere, or use different focal lengths/field of view such as ultra-wide-angle or telephoto. When you compose your scenes, you want to think more like a movie director/cinematographer instead of just a person sitting in the middle of the audience looking straight on at a stage. Think about how you could placed the camera high or low angles for more drama and visual interest, or how you can tilt the camera for a sense of motion/dynamic action, or how you can push the camera very close to certain subjects in your scene so the viewer is right there next to the character, etc.
Another common issue I see–and this is one that even troubles a lot of advanced artists–is unintentional tangents, sharing of edges, splitting the apex, etc. When you have the contour edges of your shapes touch each other (sharing an edge), it decreases the clarity of the hierarchy of your scene’s z-depth. When two contour edge touch, you can’t easily see which one is closer. What you should do instead is to overlap, so we can clearly see which one is in front of the other. Separating them so they don’t touch is also a good solution, so that the contour silhouettes of your subjects read more clearly instead of blending together. There are weird tangents where one shape connects to another unintentionally, such as a character’s arm lining up perfectly with a tree branch of similar thickness in the background, then it creates an elongated shape that you didn’t intend. Splitting the apex is when you unintentionally bisect a shape in your composition too perfectly down the middle, creating an unnatural look. For example, a character is holding a spear vertically and it bisects a dome-shaped building in the background perfectly in half.
These are just some very typical examples, and there are a lot more. In the online workshop I teach, we dedicate an entire week of lessons to just composition. You can even see some of the students’ composition assignments on the course overview page (at the bottom of the page, in the “Student Gallery” section: http://www.cgsociety.org/training/course/becoming-a-better-artist