First, if your site is not currently using www links internally, fix that by going to Admin Panel -> Settings -> System Configuration -> "WSN Website Details:" and changing the URL to include www there.
To force a redirect for people who still land on a non-www version, go to Admin Panel -> Settings -> SEO. Make you have URL rewriting set to yes there, then enter these lines in the "Other .htaccess content" box:
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]