Determining the subject in photography is certainly not as difficult as you imagine. You can keep going until you find a subject that you find interesting. Most people will take photos of nature, people, buildings, events, models, and so on.
Through these various experiences, you can finally find what interests you in photography. Maybe you are more interested in landscape photography or model photography. But this doesn't prevent you from continuing to photograph other subjects. Apart from that, usually the obstacle is the limitations of the photo object. For example, if you are in an urban area, it is difficult to get photos of natural scenery. You can go to an area that has good nature near where you live, or you can capture the city landscape from a different angle.
With a lot of practice, you will hone your photographer's eye, which is where most people usually don't see a subject, but you can see it and make good photos. Choosing a photography subject depends on what you find interesting and the lighting conditions. Continue to develop your eye skills, continue to look for possibilities to produce good photos, and continue to be creative.