This is what I would do. 🙂
1) Create a new php page
2) In it, connect to the database:
$db = mysql_connect( $host, $username, $password );
$connection = mysql_select_db( $database, $db );
3) Do a query to match the login/username and password.
4) If there exists a row of data that matches, then login is successful
5) If there is no row of data with the same login and password, print out an error message.
You might want to do something more secure, but that's the basic things I would start with. Hope it helps! 🙂