If you don't want to read the whole thing and like to get started, this is what you have to do, it's not detailed:
A configuration of your virtual host site:
put this in apache2/sites-enabled
<VirtualHost <ip address if your outgoing ethernet card:80>
# Basic setup
ServerAdmin <your email adres>
ServerAlias testsite.site.com
Servername testsite.site.com
ProxyRequests Off
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass /
http://<the ip address that your other server has>:80/
ProxyPassReverse /
http://<the ip address that your other server has>:80/
</VirtualHost>
Hope it gives you a start, but make sure that you know what you do, for minimal: read the commands at apache's site