The Unix shell is one of the most powerful and important tools available to you. It has been around for a very long time and offers a text-based interface to most of the other tools you will use.
An excellent introduction to the Unix shell is available here:
Make sure you are very comfortable with lessons 1-3 since these are crucial for using the shell. We recommend going through all the lessons at some point to maximize your ability to use the tools at your disposal.