In case you have some experience in Java I can recommend this book:
Professional Android 4 Application Development (Wrox)
It really helped me to get started with Android development. Otherwise the best start is probably a quick introduction to Java. In German there's also a book called Java for Android, not sure if it exists in English, too.
Also for inspiration you can have a look at this:
https://code.google.com/p/simple-android-instant-messaging-application/