Learn the rules of composition so you know how and when to break them.
One of the most important concepts in cinematography is composition, the arrangement of elements within a frame. There are many ways to approach this arrangement: some filmmakers follow "rules," like the Rule of Thirds and the Golden Ratio, while others use their own taste and preferences, but the two things at the center of every compositional decision are aesthetics and storytelling. In this video, Darious Britt offers a clear and informative breakdown of composition and framing, why they are important, and how you can approach them in your own work.
As usual, Britt provides us with a damn near perfect explanation of a cinematic concept that gives many beginners a lot of trouble (Britt's video also managed to make me laugh so loud that I got my wife's attention while she was watching her novelas...a near impossible feat).
The paradoxical thing about composition is that while it's somewhat rigid, it's also quite flexible. In other words, there are definite aesthetic laws that are more or less unavoidable: a red dot is going to stand out if you put it among a bunch of black dots; the eye is going to look at the largest element in a frame first; a person's gaze is going to follow vertices, or leading lines. Humans can't help but notice and react to these aesthetic and compositional patterns because it's biological, which is why so many artists, filmmakers included, use them in their work.
But like I said, composition is also flexible. You don't have to follow the Rule of Thirds, the Golden Ratio, or any other "rule" to make your images look appealing. In fact, overuse of them can make them boring, especially if everything else—acting, lighting, set design—is subpar.
You don't have to follow the rules, but you should at least understand what the rules are so you can intentionally break them. Otherwise, all you've got, as Britt says, is chaos. In the end, the most important question you have to ask yourself when deciding on how to compose your images is, "Does it serve the story?" Story comes before beauty!