A warning text in the top of page

I´ve tested my website in diferents platforms and I can see the next text in some of them:

Warning: Cannot modify header information - headers already sent by (output started at /home/antonior/public_html/administrator/components/com_jnewsletter/language/spanish.php:1) in /home/antonior/public_html/templates/gk_blackandwhite/libs/gk.template.helper.php on line 112

I don´t the meaning but the site looks to go properly allthougt the text show a very poor look of website.

Thanks in advance.

http://www.antonio-romero.com