Using kirup basic contact form - problem

I followed the instructions for the contact form http://www.kirupa.com/html5/creating_email_contact_form.htm

However, I received this message:**

Warning**: Cannot modify header information - headers already sent by (output started at /home/uniteza1/public_html/mailer.php:2) in /home/uniteza1/public_html/mailer.php on line 20

I have no idea what this means. What did I do wring?
Thank you.