Well there are multiple ways to do shading. One thing to always do first is to pick the direction of the light.I do shade/light in two main ways. The first is to create a color palette, the colors used will depend on the environment and character. With this approach I use the solid colors and do a rough shading setup and blend the colors with smudge tool. The second approach is to use the color palette but with a low opacity brush. Sometimes you can get away using just low opacity black brush, but that can make the image lose impact.The key here is that no one way is "the" correct way. There are many, many tutorials on this stuff. The key is practice. If you go back to my early stuff you can see the progress I have made, I already see progress in your art. From your derpy to your Rainbow dash it seems you have made progress in your proportions of the characters.here are a few tuts I think might help you.Colorlightingroom lighting