If you want to redirect all non www requests to your web site to the www version, all you need to do is add the following code to your .htaccess file: Needless to say that you must be running on apache with mod_rewrite enabled.
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
Redirecting www to non-www
RewriteEngine On
RewriteCond %{HTTP_HOST} !^my-domain\.com$ [NC]
RewriteRule ^(.*)$ http://my-domain.com/$1 [R=301,L]