Choosing a subject in photography is often based on a combination of the type of things or people that you enjoy photographing as well as where you are.
Are you going to a family reunion? Are you going out for a hike and hope to see some wildlife? Are going for a walk around the neighborhood? Each of these situations will present some opportunities for great photography themes. You, as a photographer, need to identify the opportunities and take advantage of them to get those great shots.
The first thing that you need to learn are some basic photography techniques which will help you capture the best photograph of the subject that you are seeing. Once you learn those techniques, selecting your favorite subject matter to shoot will be up to you. Most photographers whether they are professional or amateurs will have a medium they like to work with. It is the same with other artists; you have painters, sculptors, sketch artists, and much more. Photography is art and therefore requires an eye for the right photograph.