What languages should I know/use?

It isn't really that important.  Use what you're comfortable with,
or what you're constrained (eg by your manager) to use.
If you're just dabbling with programming, Perl is a good choice, simply
because of the wealth of ready-to-run Perl/CGI resources available.
If you're serious about programming, you should be at home in a
range of languages.  C, the industry standard, is a must (at least to
the level of comfortably reading other people's code).  You'll
certainly want at least one scripting language such as Perl, Python
or Tcl.  C++ is also a good idea.

In response to a Usenet newbie question:

I am seriously wanting to learn some CGI programming languages
J.M. Ivler wrote some eloquent words of wisdom:
If you want to learn a programming language, learn a programming language.
If you want to learn how to do CGI programming, learn a programming
language first.
 
My book is one of the few that tackles two languages at the same time.
Why? because it's not about languages (which are just syntax for logic).
CGI programming is about programming, and how to leverage the experience
for the person coming to the site, or maintaining the site, or in some way
meeting some requirements. Language is just a tool to do so.