The Dev Guide is a practical introduction to developing applications for Android. It explores the concepts behind Android, the framework for constructing an application, and the tools for developing, testing, and publishing software for the platform.
The Dev Guide holds most of the documentation for the Android platform, except for reference material on the framework API.
The SDK can be downloaded from here Android Development SDK (from USEFUL LINKS).
After you have the SDK, begin by looking over the Dev Guide. If you want to start by getting a quick look at the code, the short Hello World tutorial walks you through a standard "Hello, World" application as it would be written for the Android platform. The Application Fundamentals document is a good place to start for an understanding of the application framework.