how to remove this error?

hi
I recently uploaded a wordpress theme on my website, check it here thebestworkoutdvds.org , I wanted to install a plugin short codes last night but suddenly an unexpected error popped up, here it is
Warning: Cannot modify header information – headers already sent by (output started at /home/workout/public_html/wp-content/themes/attitude/functions.php:2) in /home/workout/public_html/wp-includes/pluggable.php on line 896
I can’t login to wp admin, I want to know how to remove this error from theme, I can do it from cpanel. I am using a famous wp theme right now called attitude