There are several options available:
A) Go into the terminal and type "man" (without quotation marks) and you will get the "manual" of UNIX with all available commands (there exist quite a few of them )
but for me this method is confusing.
B) Get a UNIX book, a paperback doenst cost that much. Just get a UNIX reference book like UNIX in a nutshell or one similar to that
C) Look online and do a search for UNIX commands in yahoo or any other credible search engine.