One quick way to get started is by entering "info coreutils". Also, "help" gets you a quick list.
Eventually, you will want to get a book, but for starters there are lots of tutorials on the net. The RUTE guide (link in previous post) is often cited. Also, look at TLDP.org--especially: "Bash Guide for Beginners" by Machtelt Garrels.
And of course there is Google........."Linux commands" gets 41 million hits