A shadow for your character on the ground? On your character?
The basics is to think of where your light source is (or are if there are several) and to try to think of all the things you want to shadow as simple shapes (like a ball jointed doll so cylinder shapes for arm/legs and so on). You can train yourself on simple shapes, like cube, sphere and so on to learn how the light and shadows act.
The following step can be to learn about the kind of shadows to have a more 3D feel to your drawing and to make more accurate shadows (since the shapes of the body are not that simple).
You can find a lot of tutorials on shadows on the web but if you explain a little more what you're looking for, I might be able to suggest a few ones for you