thats not too bad, but youve got more than one light source in them, which leads to it looking a bit unnatural (unless thats what you intended)
basically imagine a light somewhere your page, and then from there try to imagine how it would land on the object. In the case of the graffitti youve got there, you'll have to think about where shadows would be formed as well. Other than that though, its really a matter of practice - eventually it comes naturally.
"Never have I seen a greater, or a more beautiful, or a calmer or more noble thing than you, brother" - The Old Man and the Sea - Ernest Hemingway