problem s Session

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:Program Filesxampphtdocssession.php:1) in C:Program Filesxampphtdocssession.php on line 2

<?php

session_start();

$_SESSION["username"]="kodrla";

?>

<?php

session_start();



if ($_SESSION["username"]) {

   echo "$_SESSION[username] - ";

   echo session_id();

}



session_destroy();

?>