Really, I mean read the manual or as it is termed in Linux, the man pages. This is one of the really great features of Linux. The the manual is already written and installed by default in most distributions. Learn what a command does on the command line while still on the command line.
Open a terminal and type man followed by the command you need to know more about.
The man for each command will give you all the options, switches and examples for the command. Most of these are very thorough and very easy to understand. If you need more information after reading the man page, then you can always search the web for more. When you are done reading typing a q will return you to the terminal session.
If you are someone who learns better from a book, may I direct you to LinuxCommand.org. William E. Shotts, Jr. has a created nice site and he has also written a book, The Linux Command Line, which is also available in pdf as a download or can be purchased in hard cover.
However you decide to learn the command line is your choice. But knowing some of the basics may some day save your bacon or at the very least your computer.