My tips: Stay away from the airbrush unless absolutely necessary. Stay away from the smudge/dodge/burn tools unless absolutely necessary. Don't shade with a darker tone of the same color or black, or make highlights with a lighter tone of the same color or white. It doesn't work that way in real life, and it makes your work look dull. I'm assuming you're using Photoshop because I noticed the brush spacing, lower the brush Spacing to 1% for clean lines and clean tone application (you won't have those circles on the edges).
I said not to use the darker shade of the same tone. If a surface is red, don't just use darker red for shadows. Use some other darker color, that depends on the surroundings (if the background is blue-ish, then shadows should be blueish too).
You can shade with something that's darker, just turn it around the color wheel a bit so it shifts in hue as well.
The easiest way to do this is to make a new layer and set it to 'multiply' then draw in your shadow color. Blue is the usual favorite, because when you're outside in the sun you'll get a little illumination of the shadows from the blue sky.
(I also see people do some amazing shadows with a solid hot cobalt blue now and then, but that's the advanced method.)