As I've been using UNIX for 20 years I can no longer remember how to start learning it, but you could try looking at http://www.tldp.org/

One thing that I've become quite grateful for is that in the early days of UNIX, many terminals were very slow, so the commands, e.g. ls, are just a couple of characters - very handy when using a stylus. Which reminds me that some of the early UNIX documentation is online at http://cm.bell-labs.com/7thEdMan/bswv7.html

Personally my top tip would be, don't use root access. Learn to set up your own stuff in /home/user, then you can keep track of it all. See my tips on using xterm in the Wiki for how to set up a .profile that uses ENV. To help understand how that setup works take a look at the output from typing "help", and from typing "busybox". Visit http://www.busybox.net/docs.html for more info.

If you don't have a Linux box to play with, which will have "man" and much more documentation installed, then consider installing Cygwin on a Windows PC.