So, Xamarin is now bundled with Visual Studio and licensed in such a way that you can use it for whatever you like, but that doesn't help the budding cross-platform developer get a handle on the "how". If you're looking for a head start, Microsoft has an excellent video just for you.
This session, recorded during the "Visual Studio Live! Redmond 2016" conference and presented by Microsoft's James Montemagno, provides current .NET developers with some direction on using the now-free Xamarin tools to create apps for Windows, Android and iOS:
This session will also give you the tools to determine how much existing C# code can go mobile to iOS and Android, plus help you determine the architecture necessary to support maximum code sharing and reuse. You'll also receive some guidance and best practices for handling fragmentation across and within each device platform.
It's not going to make you a master overnight, but hopefully it'll help you avoid the learning inertia that comes with picking up an entirely new library or API.
The talk weighs in at an hour and 16 minutes, so be sure to settle in before you hit play.
Go Mobile with C#, Visual Studio, and Xamarin [Channel 9 MSDN]