A good Composition is even more important than good Technique.
Bad Composition doesn’t win awards, not matter how cute your painting is.
Here there are 2 easy ways to fix that.
1- First: Observe the Masters. Devour any Art books that come you way. Most of the paintings in those pages will teach you something. Observe the negative spaces in them; how and where the main subject is placed; the contrast of lights and shadows, and how shapes connect with each other.
And do me a big favor: DO read every line in the book, not just the captions! Most authors are kind of sneaky and hide juicy tidbits of priceless information among the generic stuff.
2- Second: You get your Digital camera out every day and take a few dozens of photos of subjects that intrigue you, all from different angles. Then, later, spend some time editing and cropping your photos in your computer until you end up with just a handful of shots that look really good to you. The more photos you take and edit, the better you become at finding good compositions. Keep it up for 3 months. You will not believe how much you improve after such a short time! Compare your first chosen shots to your last best ones. You’ll be amazed at the results.
Of course, there are many other ways to learn Composition, but these two methods will give you an inexpensive head start.
Good luck to you, fellow painter!