To force your website visitors to use the www part of the domain URL (useful for search engines) you can put the following in an .htaccess file:
Options +FollowSymLinks RewriteEngine On RewriteBase / RewriteCond %{HTTP_HOST} ^domain.com$ RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]
Replace domeniu.com with your domain name. After doing this, anyone who enters http://domain.com in the browser will be directed to http://www.domeniu.com