The Basics

When trying to create fan-art you need the basic tools. (These tools change between the applications you may use, or if you use paper)

Photoshop's toolbelt.

Photoshop is one of the biggest graphics designer applications in the world, it provides you with the tools to let your imagination run wild. However the imagination is only as good as the user's ability to use the tools and vice versa.
Photoshop has a massive range of tools most of which are shown in the image below

from Top to Bottom:

Here is a quick style guide on rick and morty: