what does your xml form look like?
you will atleast need a contact.php page or something like that and add the following code (depending on whats in your form)
$to = $_POST['email'];
$from = 'firstname.lastname@example.org';
$subject = $_POST['subject'];
$message = $_POST['message'];
mail($to, $subject, $message, 'From: ' . $from);
more on PHP mail can be found here: http://php.net/manual/en/function.mail.php