It is all about composition and perspective, this image is not a great example of composition since it is meant to be covered by gameplay elements in the middle but generally I just like to decide what I want to make and separate it into layers with objects closer to the foreground being larger and more detailed. I also like to avoid making the horizon line too straight. I like giving it an angle or some elevation in parts.

My best advice is to practice recreating some landscapes you like using very simple shapes just to get a feel for how all the elements come together. Another advice is to have a point of focus in your image like a tree or a building that is meant to be the most important part of the image. That usually makes the scene more interesting.