AIDE: The Android IDE
To develop an Android app you need an Integrated Development Environment, that is, an IDE. This lets you write Java code, convert it to an app code, then run your app to test it. The first stage involves writing the Java source code. You can learn how to do this at btutor.com. The second stage involves compiling it into the machine code that makes up the app. And the third stage involves testing your app by running it in a simulated Android device or, in this case, in a real phone or tablet.
There are several IDEs that can be used. The most popular is probably Eclipse which is a general purpose computer application that can be used for app development. Another is the Android Studio software, also a computer application. A third option is to use an IDE that runs on an actual Android device. It is called AIDE (Android Integrated Development Environment). This lets you develop the app then run it on the same Android device.
Like Eclipse and Android Studio, AIDE is free, at least for a starter version. This will save and compile small apps then run them in the same device. When you get to the stage where you want to write larger apps or you want to publish them on Google Play Store then you will have to purchase an upgrade to the full version. You will also have to put up with a lot of ads in the free version.