can't use my wordpress because of Warning!

hey guys ??

I have a stupid problem …
I allready have installed my wordpress and everything worked.
But sometimes, like when I registered myself, there were warnings/error messages on the top of the site.

And now if I want to access the site with my browser I just can see everything white with this message:

Warning: Cannot modify header information – headers already sent by (output started at /users/christian-jules/www/wp-config.php:1) in /users/christian-jules/www/wp-includes/pluggable.php on line 890

What does this means? I allready looked in my wp-config.php
and one time it did work, but now I only see this message and I can do nothing ??

I need help ??

xoxo