I’d start with some paper guide – maybe Peter Norton’s Guide to Linux. In fact, I didn’t read it, but never put my hands on better written guides than Norton’s.
Then search for “wiki util linux”, there you’ll find a longer list of commands.
Then open your terminal window, read man pages of the commands ( man somecommand ) you learned, liked and decided to use in the nearest future, and, of course, try them. In my opinion, the most important part is “decided to use in the nearest future” – you know, human brain is a smart creature, keeps only useful things ;o)