How to create a video or audio chat application using WebRTC in .Net?

Step 1) Do a little research. Google is a good start point
i.e:
http://forums.asp.net/t/1705212.aspx?Audio+Video+Chat+in+ASP+NET+With+C+[^] (good links collection)
http://stackoverflow.com/questions/4624442/audio-video-text-chat-in-asp-net-app[^]
http://stackoverflow.com/questions/14373558/real-time-video-chatting-recording-solution-in-asp-net-mvc[^]
http://tech.pro/tutorial/1491/chat-application-with-signalr-and-aspnet-mvc-4[^]

Step 2) Start coding
Step 3) Compile and use the debug to solve little issues
Step 4) When you get a problem you don't know how to solve but at least you tried it, then come back and ask for something concrete with a snippet of the code giving problems

Sorry if this is not the answer you were looking for. But your question is a bit too wide to be answered at the "Quick" Answers. It is better and you get faster help if you make 10 concrete questions about concrete problems, than a big question about a "how-to guide"