need help!! please.. :'( Different User Accessing Different Form in C#

first, i wanted to say thanks for helping..

i want to create a login system where the different users access different form..

i create 2 button. 1st is the register button where that person can enter what name and password their want..

and the second button is login button.. but my problem is now, although different username and password been used, it still accessed the same form..

hope you can understand this.. i'm so sorry once more