i think you should use some Cross-Platform Solution Either HMTL, or as you prefer C#, but through some other 3rd Party Plugins and framework such as Xamarin, it's the best tool to develop Cross-platform Apps for Smart phone through Visual Studio, you can get a free license easily to start building your first apps.
But as you said you are a bigenner i guess it will be best that you only focus on a single platform and build your apps with the things you already familiar with C# and Visual Studio without any extra 3rd Party Frameworks or Plugins. and focus mainly on Windows Phone. You can read about UWP (Universal Windows App) which allows you to build Apps that can run on both Desktop and Phone.
This Link discusses all about the matter and all possible solutions:
Other than that I would really suggest that you take a look at HTML or Java frameworks, those are the future of cross-platform mobile apps.