errore session_start

[bruno86]

ragazzi mi succede una cosa impossibile
questo è tutto il codice della pagina:

<?php
session_start();
$_SESSION['num']=$id;
echo $_SESSION['num'];
?>

e mi questi errori :

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /var/www/netsons.org/agloco/pass/menu.php:1) in /var/www/netsons.org/agloco/pass/menu.php on line 2

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /var/www/netsons.org/agloco/pass/menu.php:1) in /var/www/netsons.org/agloco/pass/menu.php on line 2

perchèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèèè???