I’ve found that project-based learning works really well for me. That was how I learned Android, and that’s how Udacity tends to teach as well.

So I’m certainly a bit biased towards Udacity (I was a student AND now work there), but I do think their Android Basics by Google program is a great starting point when starting out with Android for the first time

Android Basics Nanodegree by Google | Udacity

All the courses in that program are available for free as well

If you already are familiar with Java & programming, then the Android Developer Nanodegree program might be a better starting point because it will move more quickly

Android Development Nanodegree by Google | Udacity

Again, I believe all the courses are available for free

Beyond Udacity, Google has a training page with a lot of great resources

Google Developers Training | Google Developers

I highly recommend the Big Nerd Ranch as well. They do an awesome job.

Android Programming Guide | Big Nerd Ranch