Help required in developing Messanger in C#

Question

Dear Friends

I am a new programmer in c# and made very few applications in this language. i want to develope a messanger where there is one server and multiple clients and clients can talk to each other through server. server should also be capable of collecting login info and setting up the link between clients to start chat.

could anyone please tell me the flow of information required to develope this application in the form of DFD of class diagram.
Looking forward to hearing from you soon.

Regards