You basically need a login server and a database.

User enters username/password, client sends these to the login server, then login server checks if username exists in db. if it does, server checks if password in db matches with the password the user entered. If it does, you log your user in, if it does not you show a warning.

more info here:
http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=189253&page=2

not %100 correct, but it should give you some ideas.