|Create a form, make that your main form so your code starts with this. |
Have two textboxes, one for username and another for password.
Create a table with usernames/passwords
you can make the username field a drop down if you prefer and load all the usernames in there.
when the user enters/selects a username (This must be unique) and enters the password, check the table to see if they match. If they do, unload the main form, and load your switch board. if they enter incorrect username and password then throw and error and empty the two text boxes.