How can I get rid of this error message

Warning: Cannot modify header information - headers already sent by (output started at /home/acidrain/public_html/PHPNuke/header.php:32) in /home/acidrain/public_html/PHPNuke/includes/sessions.php on line 222

Warning: Cannot modify header information - headers already sent by (output started at /home/acidrain/public_html/PHPNuke/header.php:32) in /home/acidrain/public_html/PHPNuke/includes/sessions.php on line 223

Warning: Cannot modify header information - headers already sent by (output started at /home/acidrain/public_html/PHPNuke/header.php:32) in /home/acidrain/public_html/PHPNuke/includes/page_header.php on line 492

Warning: Cannot modify header information - headers already sent by (output started at /home/acidrain/public_html/PHPNuke/header.php:32) in /home/acidrain/public_html/PHPNuke/includes/page_header.php on line 494

Warning: Cannot modify header information - headers already sent by (output started at /home/acidrain/public_html/PHPNuke/header.php:32) in /home/acidrain/public_html/PHPNuke/includes/page_header.php on line 495