Id say the best thing to do is set yourself a challenage or a task that you would like to know how to do and dont stop till youve found out how to do it!
If you want to get into the CLI a bit then try doing some of the stuff that you would usually do via a GUI in the CLI.
There isnt really all that much to it, you will pick up some of the CLI stuff when you need it but dont go and try to learn a book of commands that you'll never use.
So just remember that there isnt any part of your system that you can't customise and have fun!!