RewriteEngine On

RewriteCond %{HTTP_HOST} !^www.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}%{REQUEST_URI} [R]
#TO ACTIVATE CODE, REMOVE THE POUND SIGNS #RewriteCond %{HTTP_HOST} ^www.
#RewriteRule ^(.*)$ http://yourdomain.com%{REQUEST_URI} [R]