how to handel session in wcf

i want to do an applcation to know about session.
there sould be 2wa connection with client to server(req-reply) ..
my criteria is lik ths:
one client when connect to server..server should send a reply..and at the same time ,if any other client wil send request.It should wait until the first client finish and then the next thread should start.
how to  do this..