Well obviously you'll need the Apache package (httpd), just use the latest version available in Slackware 12 (possibly updated by now).
After that, it depends what you want to do. For example, if you want to support PHP scripting you'll need PHP, if you want to run a dynamic site you'll need MySQL or PostgreSQL, etc.
Once Apache is installed, all the documentation you need is on the Apache website. You'll find that Apache as installed on Slackware is pretty close if not identical to Apache as built by The Apache Software Foundation.
If you're setting up a web server you should probably understand and use iptables (firewall) as well.