I want to learn photography in my spare time, so what knowledge do I need?

You need a camera in manual mode. Is SLR the second best?

You need to know the influence of light and shade on a photo, and what kind of effect can light sources from different angles show, so that you can better master and use light.

There is also the specific operation and function of the camera can bring you what kind of effect. For example, high-speed shutter improves the clarity and stability of photos, but in the case of insufficient light, it is necessary to increase the aperture or improve the iso to match it. That's why you need a camera that can adjust the manual shift. Only in this way can you feel the different effects of different settings more intuitively, instead of throwing all the programs to the pitifully small processor in the camera to operate, not to mention that it has no human brain and will not understand that you need it.

As for the way of learning, you can read books or go to forums, as long as it is convenient for you. After learning, you must always practice and accumulate experience.