What are these errors and how can I get rid of them?

I'm getting errors coming up on the sign up page when using membership.

Here is a screenshot:

https://dl.dropbox.com/u/13675289/membershiperror.png

Warning: Cannot modify header information - headers already sent by (output started at /home/tammie/public_html/royallancasterslondon/wp-content/themes/bretheon/header.php:13) in /home/tammie/public_html/royallancasterslondon/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /home/tammie/public_html/royallancasterslondon/wp-content/themes/bretheon/header.php:13) in /home/tammie/public_html/royallancasterslondon/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information - headers already sent by (output started at /home/tammie/public_html/royallancasterslondon/wp-content/themes/bretheon/header.php:13) in /home/tammie/public_html/royallancasterslondon/wp-includes/pluggable.php on line 682