One of the best Android development resources available is of course the Android developer site, developer.android.com
The site contains a development guide that provides a number of tutorials on Android development. They show you how to setup a development environment using Eclipse and the Android Development Toolkit and attempt to explain many parts of the operating system and the SDK. For an overview of most features they do a really excellent job.
The other often often looked feature of the Google SDK (judging by some of the questions on the forums) is the javadoc comments in the SDK reference section of the site. There are many useful and often overlooked bits of information tucked away here. We would recommend that whenever you’re working with a particular class or feature that you should check the related javadoc because you might find it extremely useful.