/ic/ - Artwork/Critique

I really don't have an understanding of how to place and use a light source and cast shadows. The thing I draw most often is cars and i focus more on just making it look like shiny metal than i do consciously thinking about how light falls on the object. i have no idea what im doing at all.

Generally speaking surfaces that face up are lighter and surfaces reflecting the ground are darker obviously but left to right i have no idea. i never understand how people figure out how to shade and highlight the hood or the roof or anything. i just kind of smudge it all around until it kind of blends but im sure there is an actual method for figuring this out.