I assume you have enabled SSL so the site is accessible via HTTPS.
Assuming you haven't changed the configuration, go ahead and type
This would remove the symlink named 000-default, which points to the default vhost found in sites-available. If you have altered the config, make sure to remove any vhost listening on *:80.
This first step is kind of straight-forward. You want Apache2 to stop listening on port 80, you remove any vhost on that port. But that's not all.
Open up ports.conf
Comment out (put # at the beginning of the line)
Go ahead and restart the web server and load the changes
service apache2 restart
Now Apache2 should listen on port 443, and 443 only.