session problem

[eluser]dimis[/eluser]
I use the session class of CI but I have the message
Severity: Warning
Message: Cannot modify header information - headers already sent by (output started at /home/www/greekchess.freehostia.com/index.php:1)
Filename: libraries/Session.php
Line Number: 315

How can I fix it?
I made a cms with session variables and now it can not work!
Dimis