The online Android Developer Guides can be quite useful, particularly because they are the most up-to-date learning resources and they can be found at: https://developer.android.com/guide.
The Google Developers homepage can be a useful resource for news relating to Android and Kotlin as well (since Kotlin is recognised officially by Google as a development language for Android Apps. This can be found at: https://developers.google.com/.
The online documentation and reference for Kotlin is probably more important that, for instance, online documentation for Java since the language is still quite new and changing quite rapidly. This can be found at: https://kotlinlang.org/docs/reference/.
For a quick experiment with Kotlin syntax, you can use the online compiler which is found at: https://repl.it/languages/kotlin