Many of my beginning students are overly concerned about their composition skills.
I've found that most photographers have an innate sense of composition.
Those who learn to identify the available compositional tools—when they're behind the camera—will do even better.
Composition is the arrangement of the visual elements of the photograph.
1) Geometric elements, such as lines, shapes, and curves
2) Contrasts of tone, color, light, in-focus/out-of-focus areas, and geometric elements
3) Repetitions of tone, color, light, in-focus/out-of-focus areas, and geometric elements
4) Being in-balance or out-of-kilter, between tones, colors, light, in-focus/out-of-focus areas, and the geometric elements
If any of the above are near the edge of the photograph, or are in a corner, they will attract more attention from the viewer.
A photograph can, occasionally, imply something that's outside the frame.
The photograph can have more depth if items are placed in the foreground, middle ground, and background planes.
The goal of composition is to convey to the viewer what you want them to see and feel.
Everything in the frame should be making a contribution.
When the composition works, all of the elements have a tension with each other, which relaxes the viewer's eye.
He or she can absorb the photograph without distraction.
Study your pictures.
Self-criticism is often easier when the pictures are not fresh.
Objectivity increases with time.
Study the composition of other photographers also.