Not the best way to find an open source project to contribute to but you can check trending per language https://github.com/trending/csharp or the top starred repos that are C# https://github.com/search?l=C%23&q=language%3AC%23&ref=advse....

I'd be much easier to find a project you use or know about so you have a lot more context into the project, it's usage, documentation, etc.