VS 2015 is required for developing Xamarin.Forms. If you do not already have it installed, you can download it here. VS 2015 Community is completely free. If you are installing VS 2015 for the first time, select the "Custom" installation type and select the following from the features list to install:
- C#/.NET (Xamarin v4.0.3)
- Universal Windows App Development Tools
- Windows 8.1 and Windows Phone 8.0/8.1 Tools