Depending on your interest there are a few great places google has provided to start gathering information on the Android operating system. A good place to start is obviously android.com.
If you intend to develop market applications with the Android SDK for platform you should check out developer.android.com. This is a great place to start learning the tools, tutorials on app development, and how to finally your application on the Android market. You can check out the market at market.android.com.
If you want hack on the platform itself look at source.android.com. Here you can find links to the git repository and instructions on how to build the entire android open source project. They also have a great google discussion group at groups.google.com/group/android-platform.
