A good tip to remember is that your object is dark to begin with, until you add a light source.
(you don't actually have to draw the light source, but imagine where it's coming from)
This light shoots out like a gun from the light source.
Any part of your object that the light can hit will be lit. Any part of your object that the light cannot hit will remain dark.
Light will never originate from the center of an object and grow darker towards the edges, unless your light source is directly in the center of the object.
(the white dots are light sources. on the left, we see how many light sources there would have to be to get it lit how you have it. on the right, we see how one light source would light your man.)
Anyways, this is a very basic concept of lighting.
