Session problem

Hejsa.
Jeg får denne session warning.

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at index.php:20) in login_success.php on line 2

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at index.php:20) in login_success.php on line 2

Scriptet ser sådan her ud:
<?
session_start();

echo "Welcome ". $_SESSION['first_name'] ." ". $_SESSION['last_name'] ."! You have made it to the members area!<br /><br />";

echo "Your user level is ". $_SESSION['user_level']." which enables you access to the following areas: <br />";

if($_SESSION['user_level'] == 0){
    echo "- Forums<br />- Chat Room<br />";
?>
<?php
}
if($_SESSION['user_level'] == 1){
    echo "- Forums<br />- Chat Room<br />- Moderator Area<br />";
}
echo "<br /><a href=mail.php>Mailbox</a>";
echo "<br /><a href=retkode.php>Change Code</a>";
echo "<br /><a href=index.php?page=logout>Logout</a>";


?>

håber at der er en som kan hjælpe..