If you truly want to become a better photographer you have to practice. After reading the post about understanding exposure I suggest you try the following. Get your camera and a lens (anyone will do) and your camera manual (unless you know how to change settings) and head outside during the day. If you have a tripod bring that as well for convenience.
Set your camera to Manual exposure mode (“M”) . Turn off Auto-ISO (if your camera has this feature, not all cameras have it). Set the shutter speed to 1/60, the aperture to f/5.6, and the ISO to 200. Now take a picture of something which catches your eye. Have a look at the back of your camera. Chances are that the picture is either too bright or too dark. That is okay. Now comes the fun part.
Change the shutter speed to 1/125 (leave ISO and aperture as they are) and take another picture. The picture should half the brightness as the previous one. Now change the shutter speed back to 1/60, change the aperture to f/8, and take another picture. The picture should have the same brightness at the previous one, since you have taken an image with the same Exposure Value (combination of shutter speed, aperture, and ISO). Now change the shutter speed to 1/60, the aperture to f/5.6, and the ISO to 400. The image should be four times as bright as the previous two images and twice as bright as the first image.
Play with these three settings for a while. Try to go crazy and set the shutter speed to 1 second (1″) or to 1/4000 and see what happens with the pictures. Change settings back and forth until you get an image which looks properly exposed to you. When you have reached a proper exposure, you are ready to move on.
Severely underexposed flower
Underexposed flower
Properly exposed flower
Overexposed flower
Severely overexposed flowed
Change the exposure mode to Aperture Priority (“A” or “Av” on most cameras). Change the aperture to f/5.6 and note the ISO setting and shutter speed. Now the camera is trying to calculate a proper exposure for you. By setting the camera in Aperture Priority mode, you have told the camera to use the aperture value you have picked but to change shutter speed to obtain proper exposure. The ISO should not chance unless the camera is in Auto-ISO mode. Take the picture and compare it to the one you took in Manual mode which you thought was properly exposed. How close are they?
Now change the exposure mode to Shutter Priority (“S” or “Tv” on most cameras). Change the shutter speed to 1/60 and note the ISO setting and aperture. Again, the camera is trying to calculate a proper exposure for you. By setting the camera in Shutter Priority mode, you have told the camera to use the shutter speed you have picked but to change aperture to obtain proper exposure. Again, the ISO should stay fixed as well.
I recommend you keep your camera in Manual exposure mode even when you take pictures of friends and family. Yes, you will get many bad shots to begin with but soon you will learn how to see which settings are appropriate in any given situation. If you pick for example Aperture Priority you put your brain in the back seat and the let the camera drive. It is good to know that the camera can drive, but for now you should take control and choose settings manually.
One last thing. Now go inside and repeat the exercise. What do you expect will change? Why?
When you have completed the exercise read the post More about exposure.