setting up a my sql cant get auto email

when i fill out the form on my site it will not send the auto email to the email address i put in the form, if you know what i mean its ment to send a email so people can press a link to make there account work

here is the code for my register.php

[PHP]

<?

include 'db.php';

// Define post fields into simple variables
$first_name = $_POST;
$last_name = $_POST;
$email_address = $_POST;
$username = $_POST;
$info = $_POST;

/* Let's strip some slashes in case the user entered
any escaped characters. */

$first_name = stripslashes($first_name);
$last_name = stripslashes($last_name);
$email_address = stripslashes($email_address);
$username = stripslashes($username);
$info = stripslashes($info);


/* Do some error checking on the form posted fields */

if((!$first_name) || (!$last_name) || (!$email_address) || (!$username)){
echo 'You did not submit the following required information! <br />';
if(!$first_name){
echo "First Name is a required field. Please enter it below.<br />";
}
if(!$last_name){
echo "Last Name is a required field. Please enter it below.<br />";
}
if(!$email_address){
echo "Email Address is a required field. Please enter it below.<br />";
}
if(!$username){
echo "Desired Username is a required field. Please enter it below.<br />";
}
include 'join_form.html'; // Show the form again!
/* End the error checking and if everything is ok, we'll move on to
creating the user account */
exit(); // if the error checking has failed, we'll exit the script!
}

/* Let's do some checking and ensure that the user's email address or username
does not exist in the database */

$sql_email_check = mysql_query("SELECT email_address FROM users WHERE email_address='$email_address'");
$sql_username_check = mysql_query("SELECT username FROM users WHERE username='$username'");

$email_check = mysql_num_rows($sql_email_check);
$username_check = mysql_num_rows($sql_username_check);

if(($email_check > 0) || ($username_check > 0)){
echo "Please fix the following errors: <br />";
if($email_check > 0){
echo "<strong>Your email address has already been used by another member in our database. Please submit a different Email address!<br />";
unset($email_address);
}
if($username_check > 0){
echo "The username you have selected has already been used by another member in our database. Please choose a different Username!<br />";
unset($username);
}
include 'join_form.html'; // Show the form again!
exit(); // exit the script so that we do not create this account!
}

/* Everything has passed both error checks that we have done.
It's time to create the account! */

/* Random Password generator.
http://www.phpfreaks.com/quickcode/Random_Password_Generator/56.php

We'll generate a random password for the
user and encrypt it, email it and then enter it into the db.
*/

function makeRandomPassword() {
$salt = "abchefghjkmnpqrstuvwxyz0123456789";
srand((double)microtime()*1000000);
$i = 0;
while ($i <= 7) {
$num = rand() % 33;
$tmp = substr($salt, $num, 1);
$pass = $pass . $tmp;
$i++;
}
return $pass;
}

$random_password = makeRandomPassword();

$db_password = md5($random_password);

// Enter info into the Database.
$info2 = htmlspecialchars($info);
$sql = mysql_query("INSERT INTO users (first_name, last_name, email_address, username, password, info, signup_date)
VALUES('$first_name', '$last_name', '$email_address', '$username', '$db_password', '$info2', now())") or die (mysql_error());

if(!$sql){
echo 'There has been an error creating your account. Please contact the webmaster.';
} else {
$userid = mysql_insert_id();
// Let's mail the user!
$subject = "Your Membership at Digital TV Donegal!";
$message = "Dear $first_name $last_name,
Thank you for registering at our website, http://www.irelandwebdesign.biz!

You are two steps away from logging in and accessing our exclusive members area.

To activate your membership, please click here: http://www.irelandwebdesign.biz/activate.php?id=$userid&code=$db_password

Once you activate your memebership, you will be able to login with the following information:
Username: $username
Password: $random_password

Thanks!
The shane

This is an automated response, please do not reply!";

mail($email_address, $subject, $message, "From: MyDomain Webmaster<admin@mydomain.com>\nX-Mailer: PHP/" . phpversion());
echo 'Your membership information has been mailed to your email address! Please check it and follow the directions!';
}

?>
[/PHP]

were the form is its www.irelandwebdesign.biz


what would i be doing wrong do i need to put a email address in the php scrip somewere or anything?