When do you want to?
Always? A mod rewrite in your .htaccess might be the best route:
RewriteEngine On
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^/(.*) https://%{SERVER_NAME}/$1 [L,R]
or you could probably add code to your globals file to do it with php like:
if($_SERVER['HTTPS'] != 'on'){
$url = "https://".$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF'];
header("location: ".$url);
}