My site works without www but not with www

I've a question that my site work without www and when I try with www it shows server not found.

Help will be appreciated.

Regards

Please redirect one to another. You can do it by modifying your .htaccess

#RewriteCond %{HTTP_HOST} !^www\.yourdomain\.com
#RewriteRule (.*) http://www.yourdomain.com/forums/$1 [L,R=301]

Look for the above line. Uncomment the lines by removing # from the begininng and change yourdomain.com to your needs.