session_start()

i have a problem.. can anyone help me?
i login to my site then, in my site appear this lines..

// Check if session is not registered , redirect back to main page.
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /srv/disk7/785830/www/schoolcrime.leadhoster.com/login_success.php:2) in /srv/disk7/785830/www/schoolcrime.leadhoster.com/login_success.php on line 3
Login Successful

here is my codes in login_success.php

// Check if session is not registered , redirect back to main page.
session_start();

if(!session_is_registered(myusername)){
header("location: login2.php");
}
?>

Login Successful

whats wrong with that?

thanks