My mental rundown when setting up a shot
I wanted to post what my mental rundown is when I'm taking a photo to try to help you some (and this is just what I do its not necessarily what EVERYONE should do:
I start with aperture. I am thinking..one subject or multiple? How far away is my subject? If close and one subject I start with a low aperture. Aperture is the opening in your lens so the smaller the number the more blur surrounding the focus area. If I am doing all three kids or a family, I would bump up my aperture so all of them will be in focus.
Then I move to shutter speed. Is my subject moving? Are we inside? If its my kids, then yes, they will be moving so I usually don't go slower than 1/125 especially inside. If you went slower you get more light but that also means more blur because your shutter is open longer letting in more light. if they move their arm or leg suddenly...you will get blur. If the subject isn't moving I can go slower than that to 1/50. If I am outside I would bump it up to a faster shutter speed to let less light in (if its sunny).
Last thing is I change my ISO. If I'm inside and its daylight, I usually have it around 400. At night, with flash I have to bump up to 800 or 1000 (higher if I don't use flash which I rarely do). In bright sunlight your ISO would be low...around 100.
If you read some things online they do all sorts of math trying to figure these out..I think you just start to know what your settings need to be eventually and you just need slight adjusting depending on the situation. I still do that when I take photos, if I'm in a certain situation I know where to start at least but with each photo I adjust as I go. Just keep thinking...is my photo dark? What do I need to do to get more light? (bump up ISO or slow shutter speed if you can) Is my photo too light? Then you need less light coming in so lower ISO or faster shutter speed depending on your situation.