Check what web server software you have installed. The default on most
distros will be apache, but again like most distributions ArchLinux also supplies lighttpd and nginx so it depends on which one you picked.
Check the config files of the installed server software. This will likely be in /etc/<softwarename>/, so /etc/apache/ or /etc/nginx or /etc/lighttpd depending on 1).
Somewhere in said configuration there will be a header stating the default document root for the default host. As @Rainer pointed out, /var/www is the most commonly used standard location, but it never hurts to make sure. Homework for this step: Lighttpd docs, NginX docs, Apache docs depending on which software you installed