You need to know three things…

ISO, Aperture, and Shutter Speed. These three settings will help you set the exposure of your image (brightness).

ISO

Changing the ISO changes the sensitivity of your camera’s sensor. A higher number means the sensor is more light sensitive and will produce a brighter image. Increasing the ISO will also increase the amount of noise within the image. To avoid noise use the other two settings to help reach a desired exposure.

Aperture

Changing the aperture changes the amount of light allowed into the camera. The aperture is read in f-stops. An f-stop with a smaller number means more light is being allowed into the camera. This will produce a brighter image. Changing the aperture also changes the depth of field. An f-stop with a smaller number will have a shallower depth of field than an f-stop with a larger number. If you want more of the foreground and background to be in focus then use an f-stop with a larger number for a deep depth of field. But if you wish for only the subject of the photo to be in focus then use an f-stop with a smaller number.

Shutter Speed

Changing the shutter speed will change the amount of time the shutter doors of the camera will remain open while taking a photograph. A larger number means the shutter doors will remain open longer resulting in a brighter image. Shooting a photograph with the shutter speed set to a larger number will create more motion blur than when the shutter speed is set to a smaller number. I am about to state something that may appear obvious to some but I have seen plenty of people make this mistake. 1/125 is a larger number than 1/1000. I am stating this because most shutter speeds you will use will be read in fractions of a second.