Project Siena is a Windows 8 modern UI application which can help you to create your own windows 8 applications with out programming. This application is still in beta but you can develop application with this.
Just install the application from the windows store and you can create a Windows 8 Modern UI applications of your own. It is easy to use this application.
Before starting you can read this help topic article, which will guide you for your windows application development. It will show you how to create, edit, save or publish your application.
Read more about this application. Project Siena official