In the camera viewfinder is a reflected light meter that is used when in manual and semiauto modes. Watching it as you adjust shutter speed and aperture is how you know what settings you need, to get a correct exposure anyway.
There are many combinations of shutter speed, aperture, and ISO (exposure triad) that give the correct exposure for any particular scene, but only a couple of those will be the right
artistic combination of the exposure triad.
Some of those artistic considerations are: how your shutter speed will render motion (see
www.cambridgeincolor.com), how the aperture select will affect depth-of-field (see
www.cambridgeincolor.com), and how ISO will afect image noise (see
www.cambridgeincolor.com).
Don't forget to use search engines for finding some of this information on your own, because when you do, you discover things you didn't know, you didn't know.