How to set up OS X server for serving static files over HTTP?
You do not need OS X Server to serve basic files. The non "server" version of OS X comes with a fully functional apache environment pre-installed. OS X Server is only for more advanced server features which are generally only useful if you're a sysadmin for small business.

To enable the pre-installed apache you just go into system preferences -> sharing and tick the box for "web sharing". Now apache is running on port 80, available to anyone who can access the mac.

You can configure it by editing httpd.conf (for example PHP is installed but disabled - just search for "php" in the config file and un-comment it, then restart apache. htaccess files are also disabled by default). Server Fault is the best place to ask questions about editing httpd.conf.

Since you installed OS X server I suspect you now have two separate copies of apache installed. One configured to run on port 80 (the one in system preferences) and one on port 8000 (the one in os x server).