This is a very simple tutorial with shading. The badly drawn sun in the corner will be the source of light. You can change it up. Practice! Sometimes, black shadows looks good, especially when lowering opacity. I often use black to shade. However, that's usually not very lifelike. Although sometimes I prefer to use black, it's best to use blues, reds, greens, anything dark. It's the same with highlights. I used brown for this example.
When shading, think of what you are drawing. Think of a cat, and think of where the light wouldn't really reach. Shade the areas of the legs and face AWAY from the light source. Shading can be difficult, just practice! My shading isn't the best, I'll admit. When learning to shade, it's best to look at many different styles.
I used Cel shading for this, and then used the Blur tool to soften it. Do what works best for you.