How to let client wait before reading what the server wrote

Hello everybody ..

I'm building a VLE (virtual learning enviroment) system I'm using networkStream to communicate between tcp client and server

the thing that I want to know is that when I write something on server for a client ... the client must make action according to what the server sent
but the client side reads instructions before server write and flush it

for example if someone want to login it must check in the server for authentication and then it should provide states either wrong credentals or granted access then the client side application must show message box

but what happen is nothing happen :) :)

please I want your help ..
I tried threading ... but I didn't know how to use it well

thanks alot in advance