One way of making sure your composition is strong is to pay attention to the positive and negative spaces.
The primary subject of your photo, a person, building, toy car, whatever, is the "positive space."
Negative space is everything else. Something you see in a lot of photography is things sticking out of heads, wires across the scene you didn't see when taking the picture, and so on. This is just from paying so much attention to the subject that photographers forget what is in the background or surrounding the subject.
Exercise: take pictures of three different subjects outside. Doesn't matter what they are, a person, a car, a building. While taking the picture, don't worry about the subject, just pay attention to what is around and behind the subject.
Use the background to compose the shot - for this exercise, the actual subject is not important. If the background is not working for you, move around until it is - zoom in or zoom out to change perspective, get low, or go higher. Whatever makes the background a pleasing photo.