The first error message, the mail() function expects 3 parameters
$message = 'My message';
mail($admin, $subject, $message);
I've no idea why you got rid of $message previously. I asked where it was defined.
2nd error. I notice from the error address that mailForm.php is in fact THAT PAGE. If you delete header('mailForm.php'); you'll solve that error.
Also, what I'm trying to figure out is why you have "thankYou.php" in the forms action when you want to submit this page to mailForm.php (which is the same page you are submitting it from)
I think I understand what you did with the form action there. With the code you showed you, you merged the two pages together into one ... Just to show us.