Hi Ben,
you need to make this a setting for your entire site in the .htaccess file that resides in the root directory of your site. There are several versions of this on the web - google for "htaccess www redirect" or something similar
Example 1 - Redirect domain.com to
www.domain.com
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !^
www.domain.com$ [NC]
RewriteRule ^(.*)$
www.domain.com/$1 [L,R=301]
Example 2 - Redirect
www.domain.com to domain.com
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !^domain.com$ [NC]
RewriteRule ^(.*)$
domain.com/$1 [L,R=301]
Doing this means that entering either
mysite.com or
www.mysite.com will redirect to whichever you define as the primary. It also means you will not be hit by a google duplicate content penalty.
HTH
Paul
Noise & stuff
www.bingbangbong.com 
