Hello, welcome to PJ!

I'm glad you are interested in trying out pixel art. In my opinion, the best way to start is to do simple things, like fruit, since they are easy to draw.

This image shows how to shade a basic ball. The yellow circle is a light source, and depending on the direction of the light source, the shadow on the ball will move.

The best way to learn is to try something and then post it in the WIP thread so others can see how you work.