How can i create a network mutliplayer login?

So i have been using unity for a long while now and i finally want to try and make multiplayer game. I have made a network game before but this is my first time trying to make it with a log in. So my questions are:

1) How can i store the names and passwords?

2) How can i retrieve the names and passwords?

3) Then how can i apply that to displaying the different names on the screen for both the client and server?

I know this is a lot to ask for but i would really like to learn how to do this in c#. Thank you.