can't log in to admin site

I can’t log on to my admin site. When I try I get this: Warning: Cannot modify header information – headers already sent by (output started at /home/mynarc5/public_html/wp-admin/index.php:5) in /home/mynarc5/public_html/wp-includes/pluggable.php on line 876

I am very new to wordpress and have no clue what to do. Please help!