“Warning: Cannot modify header information”

Every time I post a new entry on this theme, it directs me to a white page that reads:

Warning: Cannot modify header information – headers already sent by (output started at /home/tiffsblo/public_html/tn/wp-includes/post-template.php:54) in /home/tiffsblo/public_html/tn/wp-includes/pluggable.php on line 850

Good thing is that it still posts my stuff, but it’s just annoying to see that all the time.