And to once again explain how important this is, let me show you a few images of two toonies ($2 coins). All I changed between them was the direction of the light.
As you see, a small change in the nature or direction of the light makes a huge difference.
And the same is true in any photo ytou make. So always ask yourself: where is the light coming from? How is it hitting my subject? How contrasty is it? What colour is it? And so on.
I bet that just asking that question will make your photos better.