What do I absolutely need to know?

If you're already a programmer, CGI is extremely straightforward, and just
three resources should get you up to speed in the time it takes to read them:
  1) Installation notes for your HTTPD.   Is it configured to run CGI
     scripts, and if so how does it identify that a URL should be executed?
     (Check your manuals, READMEs, ISP webpages/FAQS, and if you still can't
     find it ask your server administrator).
  2) The CGI specification at NCSA tells you all you need to know
     to get your programs running as CGI applications.
     http://hoohoo.ncsa.uiuc.edu/cgi/interface.html
  3) WWW Security FAQ.   This is not required to 'get it working', but
     is essential reading if you want to KEEP it working!
     http://www.w3.org/Security/Faq/www-security-faq.html
If you're NOT already a programmer, you'll have to learn.   If you would
find it hard to write, say, a 'grep' or 'cat' utility to run from the
commandline, then you will probably have a hard time with CGI.   Make
sure your programs work from the commandline BEFORE trying them with CGI,
so that at least one possible source of errors has been dealt with.