Error message about header

At the Listings page, this error message came out. What should I do? My coding knowledge very basic, so please be very detail about it. Thank you very much in advance.

Warning: Cannot modify header information - headers already sent by (output started at /home/kedaicom/public_html/wp-content/themes/twentytwelve/header.php:13) in /home/kedaicom/public_html/wp-includes/pluggable.php on line 876