If you're here, one of the following most likely applies to you: you just got a new camera and you're totally intimidated by how to use it; you've had a camera but you've been relying on Auto Mode to take your photos for you; or you got a camera awhile ago but found it overwhelming and put it in a closet where it's been collecting dust.
The good news is YOU'RE HERE! And I'm here to help you. As a former teacher and current full-time photographer, photoeducation is a passion of mine and I love sharing what I've learned with other enthusiasts. So let's start with the Back to Basics lesson about your three main camera settings that will take you from Lover of Auto Mode to Master of Manual Mode!
The three manual settings that make up the Exposure Triangle are Shutter Speed, Aperture/f-stop, and ISO. Let's break each one down, talk about what they do independently, and then figure out how they affect each other.
Shutter speed determines how long your shutter stays open, and it is measured in fractions of a second. A shutter speed of 1/30 is much slower (a longer opening) than a shutter speed of 1/4000. There are a few things to consider when setting your shutter speed. One is how much movement is happening in front of your camera and how much of that movement you want to freeze. A faster shutter speed means your shutter is open for less time, which freezes movement. So if you are taking photos of a NASCAR face and want the cars to look crisp, in focus, without blur, you need a fast shutter speed. If you want to intentionally include some blur to highlight the speed, you can go more mid-range with your shutter speed. The motion in front of the camera matters, but so does your movement while holding the camera. If you are walking or moving while taking the photo, you need to consider that as well. I personally have shaky hands, so I don't use a shutter speed slower 1/320 if I'm not using any sort of stabilization. But that's me. You might have a different minimum speed you go.
Along with determining how much movement is shown or frozen in photos, your shutter speed also is a factor in how much light is let into your photo, or your photo's exposure. A slower shutter speed lets the shutter stay open longer, which also allows in more light, and therefore you have a brighter photo with a slower shutter speed. A faster shutter speed means the shutter is not open long, so less light is let in. That means your photo will be darker than it would be with a slower shutter speed. So on a bright sunny day, a faster shutter speed is fine because there's plenty of light even with a short opening. For a darker setting, a slower shutter speed will help will in some light for proper exposure.
So when determining shutter speed, you need to account for both the amount of movement happening AND the amount of light you have available with consideration for how much light you need to properly expose your photo.
The terms aperture and f-stop are essentially interchangeable. The setting on your camera is the same regardless of what you call it. I'm going to call it aperture just so I don't use both terms each time. Your aperture determines how much of your photo is in focus, and also how much light is let in (again, it's part of the Exposure Triangle, so it affects the exposure).
What your aperture determines in a technical sense is how wide your shutter opens when taking a photo. A lower aperture number (f/1.2, f/1.8, f/2.0) is a wider opening. The wider opening allows in more light, so a photo taken with a lower aperture is brighter than a photo taken at a higher aperture with all other things being equal. At the same time, a lower aperture (more light) means that less of your photo will be in focus, and more of your photo will have the bokeh effect. So if you are taking a photo of a single leaf on a tree, one person (more specifically, one feature on a person), or any isolated subject/subject's feature, a low aperture is great because it highlights that one element and allows the rest of the photo to have the artistic exponential blur effect.
On the flip side, a bigger aperture number is a narrower shutter opening, which lets in less light. That inherently makes your photo darker. But it also means that more of your photo is in focus. So if you are taking a photo of a group of people, or a large landscape, and you want more of your photo in focus, a higher aperture number is ideal. But this also means that less light is let in, so you might have a darker exposure.
I personally use ISO as the balancing setting after deciding on both shutter speed and aperture. ISO determines your camera's sensitivity to whatever light it available. A low ISO means your camera is NOT very sensitive to light. This is ideal when there is plenty of available light, so your camera can make do with the light it has. A higher ISO is used when there is not a lot of available light and you need your camera to be as sensitive as a possible to that light so that your camera is using it. But something to definitely be aware of is that a higher ISO, especially in crop sensor cameras, introduces grain into photos. This in and of itself is not necessarily a bad thing, but some photographers are very wary of grain, so it is something to know in advance.
If you are more of a visual learner, hopefully this graphic helps you a little. Each action that changes the lighting also has another effect to be aware of. Aperture affects exposure and depth of field/amount of your photo that is in focus. Shutter speed affects exposure and how much movement is shown or frozen in photos. ISO affects exposure and how much grain there is in the photo. Once you learn how all three of these settings work together, you will be able to master shooting in manual! But don't pretend it doesn't take practice. IT DOES. I still practice every time I'm shooting. And that's okay.