Droping by once again.
I like the progress you have done with the light beams. I also suggest you read up on what x3Darkie is talking about (no one will ever be a master of this subject). Light is tricky, and there is ton of stuff to learn about it. AND! It is the first step towards shadows.
There are tons of ways to do light/shadows, especily with drawn art. Your style is more cartoonish then realistic so I am guessing that is what you are going for, in that case it is important to remember that "less is/(can be) more".
Don´t over do shadows or light.
For instance, you are talking about shadows on the image but let me give you 2 examples taken straight from google:
http://www.gatheryourparty.com/wp-content/uploads/2013/05/snow-white-example.jpghttp://i.stack.imgur.com/VAvkU.jpgIf you look at the image from snowwhite you can see that there is only 1 light and 1 other tone, there is no shadow tone, the whole light/shadow is just 2 colors, one dark and one bright.
The same thing can be said about the scoobydo image. It clearly shows just 2 tones, light/dark for each color.
I suggest you try to light your image 2 times, use the flat you have, in one you add just the highlight and in the other you add just the shadow then compare the three (yes three images since you will have flat with light, flat with shadow, and also if you have all layer in one image flat with both.) and test for yourself what fits best with your type of drawing.
Remember that there are alot of do and don´t when it comes to shadows and lightning, but in the end, it is you, the artist that is the judge of what the final piece should look like.
I might also recommend, if you have the time to spare, that you pick up a 3d program, say blender3d, not to go 3d modeling but to learn about light.
Alot of 3d programs have alot of advanced ways to render and display light and it might help give a good insight on how it works, especily the bending and the reflection of light.
(That or you can pick up some colored light bulbs and a art-dummy at your local ikea, but blender3d is free!)