Little Success With Plugins

I’m learning WordPress, little experience; I do not write code. A huge problem is that my Plugins do not work and / or are unstable.

The two or three Plugins that are default and simply need activating do work; most like I’m doing something wrong in installation process.

A common occurrence is an error message like this — — (Warning: Cannot modify header information – headers already sent by (output started at /home/content/91/5725391/html/wp-content/plugins/statistx/statistx.php:165) in /home/content/91/5725391/html/wp-includes/pluggable.php on line 868)

Can someone please give a guy who does not write code a clue as to what this malfunction may be and how to fix it?

Thanks — — Tom