In the terminal, run
You should see something like this...
Run the ls command on each of those directories to see a list of commands. For every command there look at it's associated manual page (see man command by typing "man man" in terminal) and/or help documentation. For example, for ls you can do the following.
You can use the 'q' key to quit the manual pages.
Do that for every command in the system and you will become extremely well acquainted with available commands. You could also see this thread
where I recommend reading for another user.