see this thread for details on troubleshooting the headers already sent error:
http://www.webassist.com/forums/posts.php?id=19285
the key to locating the problem is in this part:
(output started at ... /checkout.php:11)
it means that the output causing the problem is somewhere between line 10 - 12. it is generally blank space or line between a closing php tag: ?> and an opening php tag <?php