Hello! Alright, let me see… In lowlight situations, it is always best to have a lens that has a wide aperture. In my case, I always bring either my 50 f/1.4 or my 24-70 f/2.8. Having wide aperture helps me avoid using the max ISO of my camera (using max ISO causes too much grain and sometimes you don’t want that).
However, if you don’t want too much bokeh or blurring of the background, you can combine a narrower aperture + high ISO. Whatever floats your boat, as they say. Experiment with it and see if you like the result. I try to avoid going lower than 30 for shutter speed because my hands are shaky. If you are blessed with incredibly steady hands (as steady as a tripod… what’s your secret?), then go for it!
As for your last question, that means your auto-focus is on. If you don’t want your camera to focus for you when the shutter is half-pressed, then turn it off. It’s on the lens. Just push the knob up to MF. You’ll have to focus on your own though, but that’s good for practice. Lowlight situations often call for manual focusing.
Hope this helps!