I want to connect my spark core using an Android app,and I want to do a simple program that allows me to turn on/off a led ?
The Spark tinker app from apple store and google play will allow you to do so without creating an App from scratch.
If you are looking for your own customized version then only the Android source code is available for now. IOS will be out but not yet.
If web app is suitable for you, @bdub has a great example for that!
If you want to build your own Android App, you can use Tinker as a good reference point for how to interact with the Core in Android: https://github.com/spark/android-app
Thank you guys It really helps me !