edit /etc/apache2/sites-available/yourdomain.com and be sure to have :

ServerName www.yourdomain.com
ServerAlias yourdomain.com

edit the .htaccess file of your drupal installation and uncomment these lines :

RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]