Depends on what web hosting server you are on. Usually your hosting control panel will have a option that you may choose from,

What happens when you do that is they modify your ht access file on your server. You can also do this yourself by inserting the following code...

Code:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
Just make sure you have a ssl or auto ssl for your website as forcing a redirect with no valid certificate will give a self signed warning.

Also make sure all resources being called in your website are being called over https... somethings things like background urls wont change if they are absolute.

If you are using wordpress there are lots of plugins that will do this for you. Force https being one.