This is what I used to start the very basic stuff. At least the first few pages are rather useful.

http://linuxcommand.org/learning_the_shell.php

Also, if you have nothing typed, you can hit tab twice and you'll get a list of all usable commands - you can the google the commands you're interested in. Some are N900-only, like the ones with "fiasco" in front of them, but most as pretty common. Just search "Linux command [name of command you want to learn about]", and there's bound to be plenty of tutorials/guides out there.