Composition "Rules"
There are no rules when creating fine art; there are only guidelines and established techniques which you can choose to advise or entirely ignore.
Don't get me wrong, many of these established techniques have merit; any person who knows of these guidelines are very much more likely to make a better piece than someone who doesn't know any at all. What's important to remember is that these "rules" can always be broken, and you should often try breaking them when photographing, just to see how it'd turn out.
I'd like to talk about the first "rule" you'll likely hear about: the "rule of thirds"
The "rule of thirds" is the guideline that you should try to align your subjects and horizon lines and any other elements in the photos along lines that span across the thirds of your frame (both vertically and horizontally). This rule is so popular that most cameras have the option to enable thirds lines overlayed onto your viewfinder.
Firstly, this is not a "rule" that you should follow religiously. It's a piece of advice, one that is good to know of—it's good to know that you don't always have to align your subjects in the center. But you don't always have to align your subject along a third either. Sometimes it might even look better when your image is aligned to the center.
Secondly, just because you have the option to align your composition with these third lines doesn't mean that they have to be on those lines specifically. The human mind does not have some intuitive sense to judge an element in the photo to be exactly on the thirds lines, and it's not going to explode if it doesn't align.
Generally, this "rule of thirds" is just a guideline that advises you this: Not everything has to be composed to be in the center of the frame. It's wise to experiment placing your subject anywhere else on the composition, even on the edge, if it looks nice. And it's definitely in good taste more times than not to place the horizon of your landscapes somewhere off the center, such as when you place it low to give a lot of space to the sky.
Like any "rule" in fine art, it's up to you to first understand them so that you can break them.