Error messages when adding new post

I get the following message whenever I try to add a new post or update an old post-
Warning: Cannot modify header information – headers already sent by (output started at /home/mnoutsid/public_html/testgearstore.com/wp-includes/wp-db.php:517) in /home/mnoutsid/public_html/testgearstore.com/wp-includes/pluggable.php on line 850
What do I need to do to get rid of this?
Thanks