how to create chat application like gmail..only registered users

Hi Frinds,
        i have Requirement like create chat application like gmail.i have one registration page and once register i send mail to user username and password ..then he once login in my website automatically come to chat box.in chat box visible only login users.
i am new for this functionality .
any one knows well in this scenario please share withe source code or explain the process how to do this .

Regards
venkat.S