If in your root is a file called .htaccess, then open it and add this code into it (problably after the first line, bofore other rules start) .
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]...