To Rewrite example.com to www.example.com using apache

<VirtualHost *:80>
    ServerAdmin admin@example.com
    DocumentRoot /opt/data/www/example.com
    ServerName example.com
    ServerAlias www.example.com
    RewriteEngine on
    RewriteCond %{HTTP_HOST} ^example.com$ [NC]
    RewriteRule (.*) http://www.example.com$1 [R=301,L]
    CustomLog /var/log/httpd/example.com_access.log combined
</VirtualHost>
 

Refer for more :          

https://my.bluehost.com/cgi/help/htaccess_redirect

http://httpd.apache.org/docs/current/rewrite/proxy.html