session php

[emilio_alac05]

son los errores que me crearon

Notice: Undefined variable: _SESSION in C:\xampp\htdocs\validar_usuarios.php on line 14

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\validar_usuarios.php:14) in C:\xampp\htdocs\validar_usuarios.php on line 15

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\validar_usuarios.php:14) in C:\xampp\htdocs\validar_usuarios.php on line