To see what the film will see, you press the depth of field preview button. For a given amount of exposure on the film, the shutter speed can be determined by the aperture that you set for aesthetic purposes. A camera with built-in meter can do this calculation for you. The photographer picks the aperture and the camera picks the shutter speed. Not as long as neither the camera nor subject is moving.
Longer lenses magnify the subject but they also magnify camera shake.
You'll be well advised to use faster speeds if you intend to make big enlargements from your originals. There are sometimes aesthetic reasons to use different shutter speeds. If you are taking a picture of something moving and want to show the motion, you'll need a slowish shutter speed. Another good example of photography's ability to freeze motion. ISO 400 film on a cloudy day at Venice Beach.