Error Code Help?! CSS newbie..

Hello! I was playing around trying to install a plugin the other day and may have messed with the coding.

When I try to access my admin login I get a page saying:
Warning: Cannot modify header information – headers already sent by (output started at /home/kelseyh/public_html/wp-content/themes/classica/admin/admin-functions.php:96) in /home/kelseyh/public_html/wp-includes/pluggable.php on line 876

Can anyone help me to fix the error in the line of coding? I’m a complete beginner when it comes to CSS and not sure if I know how to fix the coding.

I will be happy to post the coding for pluggable.php if anyone is able to help! Would be appreciated greatly!