Cannot send session > cache limiter - headers already sent

Cannot send session > cache limiter - headers already sent

Please tell me what the problem is. Doing everything right here on this
example. When the call index.php I get this message:
Warning: session_start() [function.session-start]: Cannot send session
cache limiter - headers already sent (output started at
/var/www/u1682519/data/www/lelbrus.by/index.php:7) in
/var/www/u1682519/data/www/lelbrus.by/index.php on line 35
Warning: Cannot modify header information - headers already sent by
(output started at /var/www/u1682519/data/www/lelbrus.by/index.php:7) in
/var/www/u1682519/data/www/lelbrus.by/index.php on line 38