Can't login to my site

Hi,
I made this site myself, with limited knowledge as you will see…when I try to login the following pops up with multiple lines…..is this an easy fix. has appeared after I updated some plugins last week. Can anyone help?
Warning: Cannot modify header information – headers already sent by (output started at /home/content/51/11774551/html/wp-content/plugins/display-text old/display-text.php:1) in /home/content/51/11774551/html/wp-includes/pluggable.php on line 883

my site is https://www.dentpower.com.au

BTW, how do I stop people sending me stupid spam in the comments area. I have managed to block it from appearing but would like to know how to stop it totally.
Cheers,
Andrew