Nothing better than man pages in my opinion. In case you don't know what those are, you can type 'man [program]', e.g. 'man rm', and it will tell you all about how to use said program.
With that in mind, all the core utilities are usually stored in /bin. So, if you have a free afternoon, do:
and then man all the program names which come up.
Maybe you're already past the basics, though. In that case, yeah, learning bash scripting would be good. I can't help you much there, though.