Here is a list of resources you need for developing apps and their download links.
- Android Studio Installer (download)
- Installation instructions for Android Studio (read here)
- JDK 8 and above (download)
- Eclipse Neon (download)
- Android Development Tools (ADT) bundle for Eclipse (download)
I recommend and use the latest stable build of Android Studio IDE. Also, Google has declared Android Studio as the official IDE for android app development.
Also , you will have to download some additional tools and more importantly an android sdk to actually start developing apps. If possible please download all the tools using the sdk manager provided with Android Studio or the ADT bundle.
If you want to download them externally click here.