In order to redirect all of the requests for yourdomain.com to www.yourdomain.com, you should set the appropriate rewrite rule. This can be done by adding the following lines at the beginning of the .htaccess file in your public_html folder
After this successful step any non www url's gets redirected to www url's
Code:
RewriteEngine on
RewriteBase /
RewriteCond %{HTTP_HOST} ^yourdomain.com [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [L,R=301]
After this successful step any non www url's gets redirected to www url's