You will need to relate the user with the customer.
mysql_query("INSERT INTO users (id, username, password) VALUES('null','".$_POST['username']."', '".md5($_POST['password'])."')");
//md5() will than be used to check that the password is correct when they login
$userid = mysql_insert_id(); //gets the users id
//customers table
mysql_query("INSERT INTO customers (custid, userid, name, email) VALUES('null','".$userid."','".$_POST['name']."','".$_POST['email']."')");
//display it all on the page
echo "Username: ".$_POST['username']."<br />"Name: ".$_POST['name']."<br />"Email: ".$_POST['email'];
//dont display password for privacy reasons
Hope that helps