make sure you have NO whitespace before you start your session. session functions need to happen in the header BEFORE anything is printed to the browser.
that is what is causing that error:
for example.. this is wrong
PHP Code:
<?
session_start();
// notice the white space above the <? tag
// this is bad mmkay?
?>
this is right
PHP Code:
<?
session_start();
// no white space or echo'ing to the browser before session is called
?>