In slack to start your apache at start up, just look in /etc/rc.d/ in there should be rc.httpd the permissions for startup should be 755. Then the daemon will start and you can configure your apache server with the files in /etc/apache. To change the content it's as you found earlier /var/www/htdocs, but you can change where the server points to in the config files. For help on the config files either get your apache server running, then go to it with a web browser (to make sure it works), and click on the manual link. Should provide you with more info than you would know what to do with.