Plug in help for newbie

I am not a web designer or programmer, but I have inherited a website for a non-profit club I lead. Our website was created using WordPress but is not hosted on WordPress. The site recently started showing an error message at the top:

Warning: Creating default object from empty value in /home/ziola1039/doubletakemotc.org/wp-content/plugins/bbpress/bbp-includes/bbp-core-compatibility.php on line 84

In the past, I had made a couple edits using the wp-admin function, but now I cannot login. I get the error message above as well as:

Warning: Cannot modify header information – headers already sent by (output started at /home/ziola1039/doubletakemotc.org/wp-content/plugins/bbpress/bbp-includes/bbp-core-compatibility.php:84) in /home/ziola1039/doubletakemotc.org/wp-includes/pluggable.php on line 1228

I read something about unplugging plug ins but I have no clue what that means or how to do it. Can anyone help this newbie? …Because our website is basically held captive until I get this resolved!

Thanks!