To force all website visitors to use the www. part of the domain in the URL (useful for search engines) you can put the following into a .htaccess file

Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^domain.com$
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]

Replace domain.com with your actual domain name.

After doing this, anyone who enters http://domain.com into the browser will be redirected to http://www.domain.com.