Well, lucky that you are starting to develop now because the tools that you need are very less , in some cases negligible. For android you only need an IDE (Integrated Development Environment) that is Android Studio that Google just announced in it’s yearly I/O this year. After years of complaining by android developers, Google finally gave them an alternative to Eclipse in the form of Android Studio. The UI is modern, the tools are modern (like introduction of Gradle Build) and the suggestive features (Will be helpful to you as a beginner) are at an all time High.Android Studio is also faster than any other IDE and the Android Virtual Device(An emulator to test your apps) finally is useful. If you are creating a game or an app that requires good graphics , you will also need a photo editing software like Adobe Photoshop or Pixelmator. Happy Developing!
Screenshots of Android Studio:-