Sonja! Glad you have joined our little art community! This is a great place to learn and share. Great teachers, great members!
If you are shadowing something like the surface of a ball, you would use it's opposite. The opposite of a color will darken it. (try a little mix and see).
If you are talking about a cast shadow of something like a tree onto the ground, you mix the shadow color for the ground. I hope that made sense.
There is a great, free color mixing video under the Paint Basket TV. You should look it up.