The following web-site and pages linked to it contains probably all the information you will ever need about the shell scripting.
http://www.shelldorado.com/links/
Call me and old fashion guy but I would by a book. My favorite is
Classic Shell Scripting by Nelson H.F. Beebe, Arnold Robbins
and if you like me use Korn Shell (pdksh with OpenBSD enhancements also branded by some people removed from OpenBSD project as Mir Korn Shell mksh)
Learning the Korn Shell, First Edition by Bill Rosenblatt (the second edition is worse and done after Bill's death).
I also like Unix Shell Programming, Third Edition by Stephen G. Kochan, Patrick Wood.
You can find on the Internet original Stephen's notes under the name An Introduction to the UNIX shell.
And of course the most important advise as with any programming is that unless you do scripting you are not going to learn it.