Everything you need to start developing apps for Android is available here on developer.android.com. You'll find everything from the API documentation and design guidelines, to information about the current device landscape and how you can distribute and monetize your app.
No two apps are built in the same way, but we've structured the information you need to build an app into the following three sections that represent the general order for app development.
Before you write a single line of code, you need to design the user interface and make it fit the Android user experience. Although you may know what a user will do with your app, you should pause to focus on how a user will interact with it. Your design should be sleek, simple, powerful, and tailored to the Android experience.
So whether you're creating an app alone or you're part of a team, study the Design guidelines first.
Once your design is finalized, all you need are the tools to turn your app ideas into reality. Android's framework provides you the APIs to build apps that take full advantage of device hardware, connected accessory devices, the Internet, software features, and more. With the power of Android, there's no limit to the power of your apps.
Everything you need to learn about the app framework and developer tools is in the Develop documentation.
Now your app is complete. You've built it to support a variety of screen sizes and densities, and tested it on the Android emulator and on real devices. You're ready to ship your app.
How you proceed depends on a variety of factors, such as your monetization strategy and which types of devices your app supports. Everything you need to get started with this process is available in the