I have created an app using Xamarin, and it works great on Testflight and Google Play beta-testing (I am not ready to launch completely yet, but nearly).
I need to know, how am I going to create the package for Windows Store (or what it is called?).
If you are developing a windows phone app, you should have a developer account.
Now if you are ready to publish (or even before) go to the Windows Dev Center and create a new app in there. There you can see the following steps to publish your app in the windows app store.
If you don´t know what your developer account is or you don´t have one, start here: https://developer.microsoft.com/en-us/windows