Help in Socket in C#

Hi all,
      I need the help and the guidance of you heroes over there. I have a created a socket application using TCPListener and it is working just fine. However, it is not as i want. Anyone telnet my machine with the port number that is specified in the TCPListener const, can get connected which means this way is not secured. So is there a way, where i control who can get connected to my application?? Or is there a way, to accept connection for those who are connected to my machine using SSH???? Please guide me how can i create a socket application for networking purpose but only selected hosts can get connected. For the meantime, i created function to ask for login and password but it is not secured as well because when the user type the password he can see what he is typing. It is not like other application where the user can see starts ***** when typing the password. I'm new to programming and your guidance is really appreciated.