In photography, the word exposure has different meanings.
With film, it refers to how many photographs may be taken on a single roll of film and is sold in 24 and 36 exposures.
When light hits the film (or a digital sensor), the act itself is referred to as an exposure.
Exposure also refers to the balancing act between the properties of ISO, Aperture and Shutter-Speed.
A good exposure means we have a well-balanced image and can see detail in the brightest of highlights and the darkest of shadows.
A bad exposure means the shadows are big black globs of darkness, the highlights have no detail in them whatsoever, or both.
Obviously, we want a good exposure.
So, how do we get it?
SETTING THE EXPOSURE
First, I almost always set the ISO to what I think the situation calls for. Then, I usually set the Aperture to control the amount of light passing through the lens or in some cases, to manipulate the Depth-of-Field.
Finally, I consult my Light Meter and adjust the Shutter-Speed to whatever brings my exposure into balance.
At this point, I take a picture and judge the results.
If my camera has a Histogram, I will look at the graph to see how my image stacks up, literally. A histogram is a visual representation of all the pixels in my image, from black to white, and however many there are.
If I don't have a Histogram, I have to rely on the camera's unreliable rear LCD display and hope for the best.
REVIEW AND ADJUST
If the first exposure looks off (too light or too dark), adjust a property or two, then reshoot. I will do this however times is necessary to get where I want to be.
Think of using a 3 tray balancing scale.
You put ISO in the first tray, which causes the scale to tip in that favor. Then you set the next property and place that in the second tray. The scale shifts again to deal with the new weight. Now add the final property to the third tray and try to bring it into balance.
You see, even though the(light-meter indicated everything was okay, it wasn't. So you adjust and try again.
In photography, there is no one-rule for everything.
One moment can be completely different from the next and it is not uncommon to change properties as needed.
HANG IN THERE
Photography does take a while to get the hang of things, I won't lie to you. That's why the old quote of your first 10,000 photos being your worst is absolutely true. Because they are.
Sure, you can let the camera do all the work for you by having it on fully automatic, but in any given scene there are anywhere from 5 to 7 different exposures that are ALL in perfect balance.
The difference will be in how the settings used for Shutter-Speed and Aperture effected the visual appearance of our photos.
Out of all those different exposures though, only 1 or 2 will be the best of the lot. They will have that certain special something that just makes them stand out from all the others.
By leaving the camera in full-automatic, the computer really has a very low probability of selecting that perfect image for us. Personally, I want the best photo I can get.
Learn these properties until they become second nature.
Yeah, it takes time, but like I said earlier, none of are born knowing this. We have to learn it, but thankfully there are so many resources available that it makes the learning curve a lot less substantial than it used to be.
Trust me, you can do this.