Trouble with flash/php tutorial +Yahoo server

Yahoo’s (small business) servers and they say they need a sendmail path indicated in the php file. I’m a super noob- would some one please help me figure out where in the heck that would go in the file? Here is the code I’m using

$sendTo = "blahblah@blah.com";
$subject = “the reply”;

$headers = “From: " . $_POST[“firstName”] .” “. $_POST[“lastname”] . “<” . $_POST[“email”] .”>
";

$headers .= "Reply-To: " . $_POST[“email”] . "
";

$headers .= "Return-path: " . $_POST[“email”];

$message = $_POST[“message”];

mail($sendTo, $subject, $message, $headers);

THANKS!