Android Studio is a fully-featured IDE built on top of the IntelliJ platform, designed specifically for developing Android applications. It has a huge suite of built-in tools that will make your life better, and help you write better applications more rapidly.
You can download Android Studio for your favorite Operating System (OS) from https://developer.android.com/studio/. The setup instructions for each operating system vary slightly, and are available on the website. This book has been written assuming an Android Studio version of at least 3.0.
Once installed, Android Studio will also need to download and install an Android SDK for you to develop your applications on. There are platform options for virtually every version of Android ever released, including emulated hardware, which allows you to test how your application will run on different hardware and Android releases. It's best to download the latest Android SDK, and one of the older versions as well to check backward compatibility (4.1 or 4.4 are good options).