Why Manual Mode?
Many, perhaps most, professional photographers shoot in Manual Mode. While cameras in automatic and semi-automatic modes get better all the time, the camera cannot guess what you want. If you want the image darker or lighter, or if you want to control the ISO, Shutter Speed, and Aperture all at once, then shoot in Manual Mode.
In three other posts, I discuss shooting in Program, Shutter-Priority, and Aperture-Priority modes. Those are good, but they still let the camera decide at least one of the three exposure variables and lets the camera decide how light and dark the image will be.
To make the image lighter, you can do three things: make the camera more sensitive to light by dialing in a higher ISO setting. Or, make the shutter speed longer (slower). Or, use a lower aperture number.
To make the image darker, you can do the opposite: make the camera less sensitive to light by choosing a lower ISO setting. Or, make the shutter speed shorter (faster). Or, use a higher aperture number.
Remember, changing the ISO controls noise, changing shutter speed controls motion blur, and changing aperture controls focus blur.
In general, changing the shutter speed one click on the dial is equivalent to changing the aperture one click. Either change means one click less light or more light gets into the camera, depending on which direction you turn your dial. Three clicks means you are changing the amount of light entering the camera by a factor of two. In photo lingo, a factor of two is called “one stop.” Most cameras change their ISO settings by a full stop; that is, one click doubles or halves your camera’s sensitivity to light.
Usually, shooting in Manual Mode means you will use the camera’s built-in light meter. That meter can be set to meter the light coming from a point, from the entire scene, or from several key points, or from only one point. You can get a good exposure by changing the camera’s sensitivity to light (ISO), the shutter speed, and the aperture until you center the light meter. If you know you want a darker image, do not center the light meter but put it on the negative side of center. For a brighter image, put it on the positive side.