Can someone explain how a login system could achieved?

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.

But i will need PHP isn´t it?

no you wont, if you do not want to. login server can still be written in gamestudio.

In fact you don't even need a mature database solution. That's only the best practical way to do that.

So my Database could also be basically a text file?

Yup. I just added to the message above.

As i added there, a db solution like mysql handles security and scalability for you. In a real world scenario you need security and stability, so a db comes in handy.

and a database in mysql can me as large as you want, cause you can handle and request each item/id/Username/Password..etc by simple htp_post request to a php script ^^

Textfiles are unsecure and as far as i remember, the readout lenght is limited..

Also, make sure you don't save the users passwords in plan text. Also, don't save them in a weak encryption with the key stored somewhere in your system.
If your server gets hacked and the passwords get released, your user will get very angry.

Instead, store and send only a (salted) hash of the passwords. Nearly every serious login systems use that.
Whenever a users logs in, he only transfers the hash of his password to the server which is compared to the hash saved there.

Thanks you for your kind help people!
I read the whole day about login and password security and so on.
And now it isn´t that hard now
My only question now is, how can gamestudio get the data?

mysql plugin, note that you connect to db only from the server and not the clients.

If you are using files instead of db it's standart file / read write operation.

If you are asking how it's suuposed get login details from clients, it's standart gs multiplayer mechanism.