For SEO and usability purposes you may want to redirect your visitors to open your site only through http://www.yourdomain.com.
Some applications, including Magento, however, require additional modifications to make this redirection work properly.
First, you should open the . htaccess file in the Magento folder. In it locate the "RewriteEngine on" line and right after it add the following lines:
RewriteCond %{HTTP_HOST} ^yourdomain.com$ [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [R=301,L]
Once you do this, save the . htaccess file and log in to the administrative end of Magento.
Then go to the System > Configuration menu and from the left panel click the "Web" button.
Unfold the "Unsecured" set of options and change the "Base URL" option from http://yourdomain.com to http://www.yourdomain.com.
Save the changes and your Magento website will be running with www.yourdomain.com only!