how to redirect website to www

Create .htaccess file in root folder of server 

And paste this code on it 

Your website redirect to www

Example if website http://atk.com  redirect to 

http://www.atk.com

RewriteEngine on

RewriteCond %{HTTP_HOST} !^www\.

RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]