In Win Forms, the client is connected to the server (and vice-versa) using the WinSock API. It is a relatively straight forward process. MSDN's documentation, Running the Winsock Client and Server Code Sample
], should help.
By the way, you should perform encryption on the data being passed between the client and the server. There are a number of ways to accomplish this but you must insure that the password passed between client and server is encrypted and that the password that is stored in the database is protected.