Hello to export/build a windows app I need to download the SDK windows 8 or 8.1?
If I want to upload an app in Windows Phone Store I build the app with Windows Phone 8.1? or windows 8 also serves to upload an app to Windows Phone Store?
When I create an app for Windows Phone 8.1 gives me an error x86 architecture and build fail.
You do not need any Windows SDKs to build for Windows or Windows Phone. I recommend you wait for the next release to work with Windows and Windows Phone builds, the Windows build system has been substantially changed and will work much better with that release. That release is coming very soon. In the meantime, the best place to work is on Android, first.