You would need to set up an Apache VirtualHost. In Ubuntu these are located at /etc/apache2/sites-available/<my site>
Your virtual host in this case could look something like this:
<VirtualHost *:80>
ServerName <myserver>
DocumentRoot /var/www # Point Apache to your web directory
<Directory />
Options -Indexes # Don't allow Apache to show a listing of the directory if someone navigates to http://myserver/
AllowOverride All # Allow .htaccess files in each site directory to be read
</Directory>
</VirtualHost>
This is the most simple set up that should get you up and running. Once you create this virtual host file in /etc/apache2/sites-available, run sudo a2ensite <mysite> to enable the site and then restart apache with sudo service apache2 restart
Navigate to http://myserver/ and you should get an access denied page, but navigate to http://myserver/site1 and you should see the correct site.