create ios, android,windows And web application in xamarin

If your main requirement is to create cross-platform application that can run over different operating systems, I am not an expect in xamarin, but you can get started by reading the guides and samples on the formal xamarin website here:
https://developer.xamarin.com

I tried a different cross-platform developing tool called "INTEL XDK" to develop imaging applications that runs on different operating systems. For more details about this tool see the following link:
https://software.intel.com/en-us/iot/tools-ide/ide/xdk
Note that I used it in conjunction with LEADTOOLS HTML5/JavaScript imaging library. You can find more details about my experience with this developing tool here:
https://www.leadtools.com/support/forum/posts/m40899-Use-LEADTOOLS-HTML5-SDK-to-Create-HTML5-cross-platform-applications-using-Intel-XDK-programming-util

Disclaimer: I am an employee of this library