Warning: Cannot modify header information

Warning: Cannot modify header information – headers already sent by (output started at /home/content/p/a/t/pattersonian/html/blog/wp-content/themes/startertheme/functions.php:11) in /home/content/p/a/t/pattersonian/html/blog/wp-includes/pluggable.php on line 890

I am getting this error message when ever i update my site, and i have seen it when accessing the site from another computer. I have no clue how to fix it. I’m using a custom theme that i am still developing. Please Help, and Thank you very much.