Unable to access my admin portal

Hi

Can anyone help me I keep getting this message when I want to access my admin portal on my site

Warning: Cannot modify header information – headers already sent by (output started at /websites/123reg/LinuxPackage21/ju/pp/er/jupperpeep.com/public_html/jadedeye/wp-content/plugins/top-posts-this-month/topthismonth.php:34) in /websites/123reg/LinuxPackage21/ju/pp/er/jupperpeep.com/public_html/jadedeye/wp-includes/pluggable.php on line 891

I understand that I need to go into the plugin php but don’t quite understand what I need to do and fear making an even bigger mistake.

Thanks
J-P