The Xamarin Developer Center is full of resources to help you get started with mobile development, with documentation on configuring your development environment, iOS, Android, Xamarin.Forms, and more. There is also code available for you to dig into, including recipes, samples, and prebuilt applications.
Open Xamarin Developer Center Open API docs
Xamarin is built on Mono, an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime. The Mono Project provides a documentation portal with information on getting started, compiling Mono, and more advanced topics.