I want to make an application that has (preferably native) versions for multiple platforms (Windows, Mac, maybe Linux, iOS). By native I mean versions made independently with native tools (Visual Studio and maybe C# on Windows, Xcode and maybe Objective-C on Mac). I want that application to have cloud-stored user-based structural data (not much; maybe XML-based? I want to minimize the server-side part, because I'm not comfortable with it) so it would be an advantage if it could be easily accessible from different network libraries. I don't need any server-side data processing. I don't want to have to learn (I don't understand well any) any web technologies; I would simply need an interface for creating and managing users and data client-side using desktop programming languages and tools.
If I were only targeting Apple platforms, iCloud would be an option.
The best I've found is Appcelerator Cloud Services (http://www.appcelerator.com/cloud), but I'm looking for other solutions.
I have virtually no budget for the project; I'm a high-school student doing programming for fun and in order not to remain idle.
What are my options?