The Common Gateway Interface
NCSA provides much helpful information about CGI here.
Also known as "parsed HTML": what they are, how they work, how to use them.
Using ISINDEX for Server-side Searches
This explains how to write a CGI script to perform simple text searches on your web server.
Matt's Script Archive
If you just want existing scripts without having to write them, try here. Some modifications are usually necessary, which Matt explains in his readme files.
If you just cant figure it out. Just go and buy the book. The best advice anyone could give you.