Mac OS does in fact use Apache. It's already installed, and that's what's running when you activate Personal Web Sharing in the Sharing panel of System Preferences.
You can configure Apache by editing /etc/httpd/httpd.conf .
You can use a text editor like TextWrangler (free from barebones.com).
Alternatively, you can use something like Webmin (free from Webmin.com) to configure with a web-interface.
There are lots of tutorials on the web for configuring Apache, but if you have specific quesitons, you might get good answers by posting back here.
In a nutshell, there's not that much to do, unless you're getting particularly fancy. If you need to run WebDAV or Python or SSL or something like that, look for tutorials on those subjects -- you'll find the steps explained very clearly. You might want to start at the O'Reilly Publishing web site.
It doesn't get really complicated (in my opinion) until you start getting into things like virtual hosting. But it sounds like you're not heading in that direction.
