Warning: Cannot modify header information

I’m not sure what to do about this…

Warning: Cannot modify header information – headers already sent by (output started at /home/mysite/public_html/wp-content/advanced-cache.php:19) in /home/mysite/public_html/wp-includes/pluggable.php on line 876

Please help.

http://ww.wp.xz.cn/extend/plugins/wp-super-cache/