Twentyeleven child theme width issue

I am new to WordPress and am trying to work through a tutorial on custom post types. I made a child of the twentyeleven theme called cooking, uploaded a new header image and changed the layout in the theme options to single column. When I did I got a message “Warning: Cannot modify header information – headers already sent by (output started at /Users/username/Sites/localhost/cooking/wp-content/themes/cooking/functions.php:2) in /Users/username/Sites/local/cooking/wp-includes/pluggable.php on line 876”. I don’t know what this means. When I view the front end of the site, the header is there but the overall width of the page is much smaller than if I were to choose a layout with a sidebar. Is this supposed to be like that and what does that warning mean? Thanks in advance.