What C# project(s) would you consider contributing to if you were a beginner trying to sharpen your skills in C# and .NET framework ? The project should be (besides all) active and not something less active and/or stagnant.
I'd advise you to get a couple projects under your belt first. Think of a simple program idea which might help you in some small way, such as a calendar or organizer. You can decide all the functionality you'd like, but have a basic basic functionality program that you can aim for first before adding new features. Trying to do too much at once can easily overwhelm you.
Once you feel comfortable with the general feel for C#, you can hope to help others on sourceforge.net et al. Most people don't have problems helping you understand the project, though they do expect you to know to do basic things in C#, meaning you can't ask them for help in that regard. That's why I would say that it would be better to have some basic experience first.
There are some interesting applications developed around the Stack Exchange API that could use some help. I think everything in that tag is actively maintained, but I'm not sure if the developer base is as large as you would prefer.
Still, you'd be dealing with Stack Overflow users, who tend to be helpful and (usually) patient with beginners.
BlogEngine.NET is a high-quality project, and what might make it even better to contribute to is that is has an extension framework so you could get started writing an extension to implement some interesting bit of functionality, rather than having to dive head-first into a complex codebase.