Android applications are written using Java, an object-oriented programming language. Android applications are commonly written in an application called Android Studio, and can be downloaded here.
If you have downloaded Eclipse, you can also download the adt-bundle to write Android applications in Eclipse. The adt-bundle can be downloaded here. PLEASE NOTE that since the release of Google’s Android Studio, Android development in Eclipse no longer recieves updates. Because of this, it is STRONGLY recommended to use Android Studio.