It shouldn't be hard to get started writing a module for DotNetNuke, but it doesn't seem like there's a whole lot of help especially when you are wanting to use C#. At Engage, we have created a Visual Studio project template which will get you started developing a C# DotNetNuke module, using a Web Application project (rather than the, in my opinion, much more cumbersome Web Site project). It is available for free on our downloads page after registering on the site.