How about right on their site, to start?

http://developer.android.com/training/index.html

Edit to add that applications are usually developed in the Java programming language using the Android Software Development Kit, but other development tools are available.

Java, here's a starting point:

http://blog.teamtreehouse.com/java-basics-for-android-develo...

Udacity has a good video course taught by the Android team at Google. Ignore the stuff about $200/month tutor support. All the materials are free.

Edit: here's a link https://www.udacity.com/course/ud853