The control of light is one of the most important topics in photography. By carefully planning your shot you can set the mood of your image in many different ways. The two main ways of controlling light on your image are the source of light itself and your camera settings.
Controlling the source of light on its most basic way is the easiest method. In a sunny day you can move your subject to a shadow to have less light. Inside your house you can turn on the lights to have more. You can also complicate things a bit and add a flash when needed.
The other method of controlling light is with your camera controls. By using the manual mode you can not only control the amount of light that enters the camera, but also the creative effects that each option has on your image.
We will start with the ISO value. ISO is not a way of modifying light but just signal gain applied to the image so it appears brighter. As this setting amplifies the noise in our images we will keep it at the native value of our camera, or the lowest represented by a number, usually 100 or 200. If it is not possible to reach the desired exposure with any of the other options we have, then we will raise this option up to the desired value.
The other two in-camera controls, aperture and shutter speed, not only influence the amount of light that reaches the sensor but also affects the look of the image.
The aperture measures the relative diameter of the lens. The wider it is, or lower the f number, the more light reaches the sensor. It also controls the depth of field on an image together with the focal length and the distance to the focus plane. This means that the more light you get, the less area in focus.
The shutter speed measures the time the shutter keeps open, thus letting the light reach the sensor. It is measured in fractions of a second (1/N) or just by that number, and the greater that number the more light enters the sensor. The drawback is that your subject must be still for a longer period of time.
Start by setting the most important on the look of the image, for example speed for a moving subject. Then adjust the other until you have the exposure you want. Make sure you are not comprising the image with this setting, for example by not having enough depth of field. If this happens dial back to the point you are safe and tweak the ISO as needed.