Let's talk about the Big Three: shutter speed, aperture, and ISO.
Photography Fundamentals 101.
I know a lot of people have a lot of different ideas about where to start when teaching someone about photography. You have to shoot in manual, or you need to take a class, or you need to shadow a professional, or whatever it may be. Some of those things are true, but the most important thing to understand is light. Photography, stripped down to the most fundamental aspects, is about interacting with light. It's also about how YOU interact with said light. Once you understand the basics of how your camera works, you can use that knowledge to get the photos that you want.
This is your camera.
Or, this is what mine looks like. I am a Canon user, and I will be referencing this during this post as I talk about the Big Three.
So this is the control displayed as a fraction on your camera screen display. It's a measure of time in seconds. Basically, it's how fast the shutter of your camera opens and closes, which is where that clicking sound comes from. The lower the number (larger fraction like 1/50th), the more light is allowed to come into the camera. The higher the number (smaller fraction like 1/2000th), the less light comes in because the shutter is opening and closing so rapidly. When you're photographing people, you typically need to shoot at 1/250th or faster to be able to capture crisp clean shots without motion blur. Sports photographers shoot even faster so they can get those action shots. Photographers who photograph the night sky will often set up their camera with a tripod, and leave the shutter open for sometimes 30 seconds or more to capture the light of stars in a pitch black night.
Aperture has to do with your depth of field. It has to do with the opening inside your lens. If you've ever gone lens shopping, you'll notice that most lens list the aperture at the end of the title. It could be "Canon EOS 50mm f/1.2". They list the max aperture, the widest the opening in the lens can get. It's different than shutter speed, though.
Basically, the smaller the number (f/1.2), the smaller the depth of field is. In the examples below, the photo of the girl with the flower has a very small point in the photo where the subject is in focus. Her eyes are some of the only things in focus in the left photo. But if you want more things to be in focus, say if you have a larger group of people like a bridal party, choose a higher aperture, like the photo on the right. The depth of field in the right photo is much larger, and you can see more of the background.
When you're purchasing lenses, ones with lower max apertures are more expensive. It's because they have to be more precise. If you want to buy a 50mm f/1.2 lens, you want it to be crystal clear when you're shooting at 1.2 (it is!). Getting useable photos at max aperture means the lens needs to be more exact, and components need to be made of glass. This makes them heavier and more expensive to get the precision.
Now for me, I use almost exclusively prime lenses (lenses that don't zoom and are stuck at one focal length) in all my work. I love the softness that I can get in my photos with prime lenses. On zoom lenses, often the max aperture you can get, even on the really expensive ones, is f/2.8. And for most people, that works just fine.
ISO is your camera sensor's sensitivity to light. The higher your ISO (16000), the more grain, or noise, you will have in your photos. It comes in handy in low light situations, though. The lower your ISO (100, if you're shooting outside on a bright day), the less noise your photos will have and the sharper it will look. For the most part, when I am doing photo sessions in the daytime, I keep my ISO pretty low, and use my shutter speed if I need to change my exposure. But for weddings that I do in barns, or sessions that stretch past sunset when I need that extra boost, I bump up my ISO, and sometimes lean into the grain I'll get in the end shot and embrace the film look.
So, now what?
Well, let's put all this knowledge together. When teaching people photography, I tell them to change their settings in this order: shutter speed, ISO, aperture.
If you're in a bright, high-light situation and your photos are coming out overexposed, change your shutter speed first to something faster, like 1/800. Then, make sure your ISO is turned down all the way. I always tell people to change aperture last, because it will then throw all your other settings out of whack. If you change the aperture to something bigger (from 2.8 to 1.8), you're letting more light in and reducing your depth of field, making your photo brighter. If you go the opposite direction, it becomes darker.
If you're in a low-light situation, I usually wouldn't recommend going slower than 1/250th of a second to avoid motion blur. So bump your ISO way up, and consider changing your aperture to a smaller number to let more light in.
I always tell people to start shooting in manual mode in a low stakes environment. That forces you to think on your feet and make decisions when you see how your photos look in-camera. If it's too bright, learn to adjust. If it's too dark, learn to adjust. Soon, you'll be able to make those decisions on the fly, and be able to take good photos in environments where the light is always changing. I of 99% of my work outside, and the sun is an ever-changing light source. It's the best way to learn!