Composition is a far from simple topic, yet also not impossibly difficult. The main reason it is not simple is that there are no rules to simply follow. Yes, that's what I said: no rules. There are some principles of composition, but they are not rules because often the great shot requires that you break them.
The average photographer follows the rules. The outstanding photographer knows when to break them and how to make their own up.
A snap-shooter records what they are presented with, generally with little or no thought to the arrangement of subjects within the frame, while a professional crafts the position of and relationships between the objects in the scene.
What we are looking for with composition is a successful arrangement and inter-relationship between the main elements of the image and with the shape of the image. But successful in what way? Not some rules imposed by someone else, like a judge, but rather in what you want to say with the image. Everything that is in the image should contribute to this message and their placements within the frame and with respect to each other should all serve to enhance your vision.
See the design principles on the right.
Furthermore, there are ideas about the placement of key elements in the image, such as the Rule of Thirds, static vs. dynamic arrangements, the Golden Section and more that can all be worked with.
A common thing we hear is that you should not put your main subject in the centre of the image. This is often good advice and placement on one of the one-third intersections works better much of the time. But there are times when a central placement is perfect. How can you tell when? By trying various subject placements and learning which ones work for you and in what situations. Then when you are shooting you listen to your intuition, which has been informed and trained by the prior practice.
There are also situations where the image works best if the main subject or subjects are put almost on the edge of the frame, moving them even further from the centre. This can be the case when you need to create a strong sense of space or sparse- ness in the image. Sometimes it is also best to substantially crop the main subject.