Learn perspective and after that, stop using it, otherwise your landscapes become too geometric, unless that is what you want. Pick two points of focus in your landscape, one is enough, but two is more interesting, the first one should be stronger though. To learn how to do this, you must then learn composition, rule of thirds, the golden ratio. Then learn geometry and understand how different shapes lead the eye into different points in the canvas. Becoming more adept at learning where to detail and where not to detail helps too. Also learn value and color contrast and how you can use them to aid your composition. Real life reference also helps, so look up images online or better yet, in person.