Developing a mobile application

You can use C# to develop applications for Android but not for free; you need to purchase either Mono for Android or Xamarin. Xamarin is very new and is not yet available. There is a lot of talk about it but very few useful details. We know that the initial developer of Mono and the Mono tools for mobile phones is the owner of Xamarin or at least the cheif developer of Xamarin. Many software developers are confident that Xamarin will be useful, very successful and is worth waiting for. Mono for Androidis available but most software developers would say that it is a big mistake to use it; most of them want their money back. All of that however is likely more than what you can afford. Actually I think there is a demonstration version of Mono for Android that you can get for free but it only works in an emulator; it does not work in a real device.

You can download and use the Android SDK but you must program in Java to use that. If you knew C++ then you could use that but I think you said you don't know C++.

If you want to use C# then choose Windows Phone 7. Most of what you learn about Silverlight for Windows Phone 7 will be useful in desktop Windows and web sites and most of what you learn about Silverlight for desktop Windows and web sites will be useful for Windows Phone 7 also. I think you can get for free everything you must have to get started with Windows Phone 7 development.