I thought this might be a good introduction to Android development. Book is from 2016, but it probably still works for getting started. Writing style is (to me at least) confusing, with odd use of the English language. It talks about how Android Studio (from Google) contains everything you need but not how to get it. When it describes how to install it, the book says you also need to download a Java environment, with a link to get that. It never really says how to install either one, just some screen shots of options you may or may not want to select with no explanation. There’s a section on “Why to use Android Studio over Eclipse ADT” that never really explains what Eclipse ADT is or why it’s relevant for anyone, followed by steps for migrating from Eclipse to Android Studio. I guess there are a lot of Android programs in 2016 built in Eclipse that you might want to use as a starting point??? I didn’t dig deep into programming examples but it looks like standard stuff. Overall it’s probably usable for learning.

Google Play has a bunch of other learning apps and guides, many were “free”.

Android Studio looks to be free, as does the Java SE environment, so costs to get started are your time and imagination. Android Studio is here, remove the extra spaces:
https: / / developer.android.com / studio