chat application with dotnet framework4.0

i want to integrate chat application in web using c#.net in which i want to answer all my client queries,i have fixed 100 clients with there clientid in my database.there will be one administrator who will give answer to client if they have any problem in real time or if administrator is offline
message will be displayed to administrator once he come online with regard to each user.
please help me i am new to chat i haven't work before on chat application how to implement this