What to do in order to avoid noises on records from DSLR for greenscreen? I am an amateur, but I would love to record myself at video and then cut the background, so I can paste myself on top of prepared slides.

Unfortunately each time I record a video there is a noise at this video.
Even it is more noisy after green background is cut.

I use DaVinci Resolver to cut green background by adding new layer and make delta keyer. I do same as they suggest in the video How to Turn Green Screen into Replacement Background | DaVinci Resolve 16 Tutorial. 

I am recording myself with Nikon D5200 DSLr at automatic mode at 720 HD format.
I bought a green screen from the shop so it is kind of proper one I guess.
The light is not well at my room, but I try to do my best. So I took off lampshade from my lamp at the top (ceiling). And I set two small lamps on both sides of me. Or the other setup I did was one small lamp under me and one small lamp in front of me. This way I got 1250 ISO at automatic mode.

I tried also to set manually lower ISO, but then colours are ugly.

What should I do to record myself properly so after I can cut green screen behind me and paste myself at the top of slides. But I would like to be shown without such a noise. Can you recommend me something in my situation?
Well, the noise has little to do with the green screen, with the probable exception that you can now compare your image vs a clean background.

The only way to reduce the noise is by having more light.

Try to open the aperture to the max, you can or lowering your shutter speed, (But if you are making rapid moves this could have issues)

Making the ISO Higher will give you more noise, so, if you can not do the previous two options, you need to buy a stronger light. Fluorescent lamps are a decent option, they are not as expensive as led lights.

If the noise is on the background, you need either put more light on it or probably have a smoother background.

Do not use auto mode, learn how to set up manual mode, so the image does not change in brightness as you move.