I've two tables (clients, admin) A person logs in by email and password, I have to see if either the e-mail and password is available within the customer or admin table. Your email are unquie so will not have trouble there. To uni work, clearly I'd possess a better database design than this but I must use what's given. I've this:

$email = $_POST['email']; 
$password = $_POST['password'];

$sql = "SELECT * FROM customer, admin WHERE cus_email = '$email'
 AND cus_password = '$password' OR admin_email = '$email' AND admin_password = '$password'";

Am not so good with Joins and so i havent attempted one about this.

Thank you for any help :)

You actually need a UNION as opposed to a JOIN here:

SELECT username
  FROM Customer
 WHERE cus_email = ? AND cus_password = ?
UNION
SELECT adminname AS username
  FROM admin
 WHERE admin_email = ? AND admin_password = ?

You need to either use placeholders - the question marks - or safeguard yourself against SQL Injection attacks. You will find functions that will help you do this in PHP.

SELECT cus_id as id, 'false' as IsAdmin 
FROM customer
WHERE cus_email = '$email' 
    AND cus_password = '$password' 
UNION ALL    
SELECT admin_id as id, 'true' as IsAdmin
FROM admin
WHERE admin_email = '$email' 
    AND admin_password = '$password'";