There are three main technical components to having correct exposure in your photographs. Once you have a clear understanding of these components, you will be able to shoot on manual mode to create the images you have in your mind.
Stands for Industry Standard Organization. This tells the camera how sensitive it needs to be to light. As a film photographer, setting the ISO is generally the first setting I enter into my camera/light meter. With film, you set it and forget it until you change out your roll of film. You then have to adjust the aperture and shutter speed next to get the correct exposure. In brightly lit rooms and outdoors on sunny days, you will need a lower ISO, like 100, 200, 400. In dark rooms with low light you will need a higher ISO, like 800, 3200. Keep in mind that the higher the ISO, the more digital noise you will get in your images, which also creates images that are less sharp.
One of the more difficult components to understand in photography. Aperture is how wide the shutter will open which will control both depth of field and exposure. Adjusting your aperture is what will give you a beautifully blurry background behind what you are focusing on. It also changes the amount of light that enters into the sensor which makes the image brighter or darker. Although its considered shooting "Wide Open" at a lower aperture, you're actually getting less in focus. When I was learning, it was explained to me like this: An aperture setting of f/1.2 means you will have 1.2 objects in focus, and everything else will be blurry. An aperture setting of f/5, means 5 objects will be in focus, and f/20 means everything will be in focus. When I photographs of a single person, I usually shoot at f/2-3 aperture. When I photograph landscapes, its usually at f/5-7.
Homework: Photograph one still object and adjust the ISO, Aperture, and Shutter Speed individually. You will quickly learn how each component depends on each other to make a correct exposure.
This controls how fast the shutter opens and closes. The faster the shutter opens, the less amount of light gets let into the camera creating darker images but sharper images. If the images are too dark, you need to either decrease the shutter speed, open your aperture, or raise your ISO. A fast shutter speed is great for fast moving subjects such as children, animals, and vehicles. The longer the shutter is open, more light is let in creating brighter but possibly more blurry images. A longer shutter speed is a great option for low light situations like wedding receptions, which can show movement during dancing.