PHP - Error al ingresar a la página "headers already sent"

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\cristian\verificar.php:1) in C:\xampp\htdocs\cristian\verificar.php on line 14

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\cristian\verificar.php:1) in C:\xampp\htdocs\cristian\verificar.php on line 16

{

  session_start();

  $_SESSION['usuario'] = $usuario;

  header("Location: index.php");

 }