host = Your DB host type. This is usually 'localhost' dbusername = The username you use to access your database with. dbpassword = The password you use in conjunction with the username to access your database.
$connection = @mysql_connect("$host", "$dbusername", $dbpassword") or die("Could not connect to the database. Reason:" .mysql_error());
Note: The bold tags are simply there to help you make sense of the code.
Refer to the $connection VAR later in your script when using queries and other stuff like that. Make sure you use the mysql_select_db(); function to select which database you are using.
May I suggest you take a peek at www.php.net/manual/ for futher references on connecting to databases.