1) First run sudo apachectl -S to find the main document root.

2) sudo pico /etc/apache2/httpd.conf

Make sure Include /private/etc/apache2/extra/httpd-vhosts.conf is uncommented and then add

Include /private/etc/apache2/vhosts/*.conf

3) sudo apachectl restart

3) If you are configured, good, otherwise follow this: Apache Configuration on Mac