This question doesn't really suit 'Quick Answer' section.
You also don't seem to have done basic search on the topic.
This[
^] would give you a start.
You would also like to check out
SignalR[
^] library for ASP.NET. Quoting first line from their site - ASP.NET SignalR is a new library for ASP.NET developers that makes it incredibly simple to add real-time web functionality to your applications.
Hope that helps you get started.