We are going to look at getting Xamarin installed on your Windows PC running Visual Studio 2017 in this recipe. Xamarin can be installed as part of a workload when Visual Studio 2017 is installed. For now, let's assume that Xamarin has not been installed and that you need to do that now, after you have installed Visual Studio. Go to the Visual Studio website at https://www.visualstudio.com/ and download the installer for the version of Visual Studio you installed.
You can also run the installer from the New Project dialog screen in Visual Studio 2017. If you collapse the installed templates, you will see a section that allows you to open the Visual Studio Installer.
You will also need to install Xcode, which is Apple's development environment. This can be downloaded for free from the Mac App Store.
Note that you will need to have an iTunes login to download Xcode and complete setting up your Mac. Chances are, if you have a Mac, you most definitely will have an iTunes login too.