how to create a login form to my game?

Unity 3d is cool stuff...

If there are 1-2 users you can even put their username/pwd in code of login form and at click of submit button you can check values [i dont prefer this, but it's an option]
If you've limited set of users, e.g., 2-3 then you can put their username/password in any config file. Keep password encrypted and your application can decrypt that

If you have more number of users, 10-2 then you can use db, use sqllite which can be shipped alongwith or LocalDb, or simple mdb.... this world has lot of options :)