If you've not installed it to your hard drive using Ubuntu's installer (there's a desktop icon for it) then you're running entirely from the LiveCD (your CD drive will spin regularly and it will seem painfully slow to open things sometimes). Please remember that any changes you make will not be saved to your hard drive while running from the LiveCD.
You can get to the command line in a number of ways but perhaps the easiest is to open a terminal window (Applications > Accessories > Terminal).
In terms of commands, there are lots of beginner guides on the internet you can just google. Here are a couple:UNIX tutorial for beginnersUNIX for beginners
Make sure you checkout the sections on redirecting text and piping. They are really neat command line tricks.
You'll notice that these are UNIX guides as linux follows the POSIX specification which makes it UNIX compatible.
Most UNIX commands can also be used in the BSDs (FreeBSD, OpenBSD, NetBSD, etc.) and there is also compatibility with the commercial UNIX variants like Mac OSX, IBM's AIX, Hewlet Packard's HP-UX and Sun's SolarisOS.
One of the forum regulars - Pudge - has written a great beginners command line guide, so you could PM him and ask if he still has a copy.
Once you've got comfortable with the command line, you'll probably want to get yourself a reference text. I use Linux in a Nutshell
. It's more of a reference text cum dictionary of linux commands, but there is also some useful text on bootloaders, shells and text editors. It's excellent value for money IMHO and I regularly resort to it for help on commands (man pages don't have the same appeal to me).