When photographing you want to have as clean of a background as possible so the viewer can focus on your subject. As you start out you most likely will not be shooting in big arenas, but places with cars, fences, and other distractions. You'll never see a great sports photo with a portapotty in the background.
You can do this by positioning yourself at a good location around a field or in a stadium based on what the background will be. It also helps to have your background far away from your subject because they will become blurred and you wont be able to make out the distraction. This way of shooting uses the playing surface as the background with tends to have very few distractions. I sometimes use a small step ladder to get above the field and shoot down. But a hill or grandstand can provide the same effect.