If you don't have an Apple ID already you can create a new one at. It may be possible to use an older version of Xcode if your Mac is not compatible with the latest version. See Apple's minimum requirements documentation A Mac that is compatible with the latest version of Xcode.To build Xamarin apps for iOS or macOS, you'll also need: A Mac with macOS High Sierra 10.13 or above.
Download Visual Studio for Mac Requirements