Keith,
Depends on the Distro. If you are using CentOS or any RHEL alternative, then typically you place the site conf files in:
/etc/httpd/conf.d/
The following line should already be in the httpd.conf file provided by Apache it may just need to be un-commented:
Include conf.d/*.conf
FYI site configuration files placed in the conf.d directory are given run priority by lower numbered files first. For example 10-example.site2.conf would be run before 20-example.site1.conf.
If you don't want to use the conf.d/ directory then you could create a directory like /etc/httpd/conf/vhost/ and place your .conf files in there. You would then simply add an includes file like so in httpd.conf:
Include conf/vhost/*.conf
If you are running Apache in a Debian or Ubuntu based distro than all site conf files go in /etc/apache2/sites-available/. Apache2 in Debian and Ubuntu require that new sites are enabled. to do this after creating your site's .conf file run:
sudo a2ensite your-site-name.conf (this is the same name you gave the conf file in /etc/apache2/sites-available)
Also in Debian/Ubuntu you need to ensure that the mod_rewrite module is enabled. To do this you type just once.
sudo a2enmod rewrite
Hope that helps.