How to redirect your website url on www using .htaccess file

RewriteEngine on
RewriteCond %{HTTP_HOST} ^websitename.com$
RewriteRule ^(.*)$ "http\:\/\/www\.websitename\.com\/$1" [R=301,L]



open text editor and create new file paste this code on your file and save this file as .htaccess 
name.

after that upload this file to website folder on your server.
then refresh website.