I agree with the answer provided above except for the part that you need a Xamarin license. Xamarin is now free and is a built-in installation available for Visual Studio 2015 Community Edition (or you can also download for free Xamarin for Visual Studio 2013).
Working with Xamarin and Sitecore might seem a bit tricky for the first time. You may find some ...
Mobile SDK for Xamarin supports Sitecore starts from the Sitecore CMS 6.6 SP1 version. Additionally, you need to install Sitecore Item Web API module v1.2 or later on the Sitecore instance.
Also For iOS and Android application development using the Xamarin platform, you also need:
Xamarin Studio 5.5.4 or later installed
A Xamarin license
You can get ...
Other than the above mention official resources, Sitecore MVP Goran has done lot of advance stuff where anyone will find useful
Sitecore has a mobile SDK for Xamarin. Following resource might be useful
Sitecore Mobile SDK for Xamarin - Xamarin Website https://components.xamarin.com/view/Sitecore.Mobile.SDK
Xamarin Website https://components.xamarin.com/gettingstarted/sitecore.mobile.sdk