Your job as a photographer is to arrange the elements in the frame into a pleasing composition that will result in a compelling image that people will want to look at. When you put the viewfinder to your eye, you distill everything in front of the camera into a tiny rectangle.
The contents of the rectangle change when you zoom in or out, when you move to the right or left, when you crouch down, or when you view the scene from a higher vantage point.
As a photographer, you choose which camera mode to shoot with, which lens to use, whether to have a large or shallow depth of field, and so on. You also choose what to include in the photograph and what to leave out. When you photograph any scene, you compose your image through the viewfinder or LCD monitor.
Many photographers include too much information in their images. When people view a photograph with too much visual information, they aren't sure what to look at and they don’t know why you took the photograph. When you compose your image, take a good look at what you see in the viewfinder and ask yourself if everything needs to be in your image.
For example, when photographing a desert landscape, does the cactus in the right-hand corner of the viewfinder add anything to the image? If it doesn’t, either zoom in or move to a different position until it’s gone. On the other hand, if you determine that the cactus stays, is it in the best possible position to draw viewer interest? If not, move until it’s in a position that’s pleasing to you.
You can also diminish the appearance of elements within an image through use of focal length and aperture. Say you’re shooting a picture of a deer in a state park. You want the deer to be the center of your attention, but you also want to include some of the background to give your viewer a sense of place.
Of course, you don’t want the background to be in sharp focus. That would distract the viewer’s attention from your subject. Your solution: choose a focal length that will include both your subject and the background and a large aperture (small f-stop number) that will blur the background so it doesn't detract attention from your subject.
Zoom in to fine-tune what’s in the viewfinder, removing elements from your composition until you’ve removed one too many and your story is no longer being told. Then recompose your picture to add the element back and take your picture. The practice of what to leave in and what to leave out of the picture becomes second nature with practice. Take lots of pictures.