Get out of the Auto mode and learn about exposure using the exposure triangle
The exposure triangle explains how the individual aspects of exposure, i.e aperture, shutter speed and ISO, affect the final exposure of the photo.
It's a useful way of describing the relationship between the three aspects of exposure. Each corner of the triangle represents one of the three variables, aperture, shutter speed and ISO. Adjusting just one of these will make the photo darker or brighter and will change the appearance of the photo based on what you have changed
Watch the film for more info on the exposure triangle