Soory, voi che ne sapete sicuramente più di me...cos'è quest'errore??

codice:
Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/virtual/site100/fst/var/www/html/controllo.php:8) in /home/virtual/site100/fst/var/www/html/controllo.php on line 8

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/virtual/site100/fst/var/www/html/controllo.php:8) in /home/virtual/site100/fst/var/www/html/controllo.php on line 8
mi esce nella pagina di controllo del login