Creating a small simple app requires you to master the secrets of:
- C# and the beautifull .Net framework
- Visual Studio
- Xaml
And for integration with third-party software / services you should learn something about:
- WCF / Webservices
- Json
Do not forget that you do not have to create commonly used functionalities by yourselve, these are probably available in NuGet packages.
When you are ready to take your app to the next level I can recommend you to master the MVVM design pattern, this will make your app very scalable