Warning: session_start() Problem, Fairly new at this!

[Permallama]

My error for my headers are:

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/content/98/8645198/html/twolves/index.php:1) in /home/content/98/8645198/html/twolves/libraries/joomla/session/session.php on line 531

I understand that the problem seems to be my website outputting before the session_start() What I am having problems understanding is what I need to change, Do I need to edit my index.php or the session.php And would it be the CSS of my header? Or a simple change, Thankyou for the help I will post either files code if needed.

[fb4php]

Do the session start before ANYTHING is sent to the browser! If, as you state, stuff like CSS has already gone to the browser you are too late!