Following rewrite rule will add www to all URL’s on your website in case a visitor didn’t type www or followed a link without www. Add this to your .htaccess file:

Options +FollowSymLinks
RewriteEngine On
rewritecond %{HTTP_HOST} ^yourdomain.com$ [nc]
rewriterule ^(.*)$ http://www.yourdomain.com/$1 [l,r=301,nc]

Syntax:

rewritecond – defines rewrite condition
%{HTTP_HOST}  – Server variable
^ – start matching
$ – end matching
[nc] – non case sensitive
rewriterule – defines rewrite rule
(.*) – match everything
$1 – append original URL part captured in (.*)
l – last rule (only omit in linked rules)
r=301 – redirect using 301 response (permanent redirect)