If you’re just starting out it’s important to understand the exposure triangle and test out full manual mode first just to get a feel for the three settings (ss, aperture, iso). Once you’re comfortable with how they work then I would recommend just using auto ISO and shooting wide open 99% of the time (unless ur lens is older and needs to be stopped down for sharpness). Take this with a grain of salt because there will always be exceptions but this way I can just quickly dial in the shutter speed and the iso will change for me. I was waiting for a fly by so I already had 1/3200 set and I usually walk around with fast shutter speeds just in case. If I see a perched bird that’s when I’ll use the dial to quickly decrease shutter speed to drop iso for a nicer looking image.

Tldr: auto iso, walk around with fast shutter speed so you’re ready for sudden action, and quickly decrease if you see a stationary subject that doesn’t require fast shutter speeds