I do agree it is a relic from the 20th century and I don't use it in any of the websites I admin, but if you really wanna do it, just login into cpanel, go to Domains and set up a permanent redirect from yourdomain.com to
www.yourdomain.com.
You can also do it via .htaccess, just add:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^yourdomain\.com$ [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [R=301,L]