Errors? Possibly useronline related?

I have a feeling this is related to a problem no one is helping with, but this error occurs when I go to change my index template:

Warning: Cannot modify header information – headers already sent by (output started at /home/katehack/public_html/wordpress/wp-settings.php:83) in /home/katehack/public_html/wordpress/wp-admin/templates.php on line 74