Membership: Warning messages comes up when a user registers

Hi, when you register on:

http://www.mchw.net/index.php/registration?action=registeruser&subscription=3

after completing all the details and clicking 'sign-up' it says:

Warning: Cannot modify header information - headers already sent by (output started at /home/mchw/public_html/wp-content/themes/green-stimulus/page-sidebars.php:5) in /home/mchw/public_html/wp-includes/pluggable.php on line 669

Warning: Cannot modify header information - headers already sent by (output started at /home/mchw/public_html/wp-content/themes/green-stimulus/page-sidebars.php:5) in /home/mchw/public_html/wp-includes/pluggable.php on line 670

Warning: Cannot modify header information - headers already sent by (output started at /home/mchw/public_html/wp-content/themes/green-stimulus/page-sidebars.php:5) in /home/mchw/public_html/wp-includes/pluggable.php on line 671

Warning: Cannot modify header information - headers already sent by (output started at /home/mchw/public_html/wp-content/themes/green-stimulus/page-sidebars.php:5) in /home/mchw/public_html/wp-includes/pluggable.php on line 866

what do i do to fix this please.