- Starts with the Coursera's course Kotlin for Java Developers (free course)
- If you like Venkat's style, you'll like Kotlin for Java Programmers by Venkat Subramaniam (video) - it's a pragmatic and funny introduction on the key features of the language.
- Have fun following the nice Kotlin Koans to dig into the bits of the language
- Explore the rich and complete official documentation at https://kotlinlang.org/docs/reference/
Then, if you like Kotlin and want to go on learning and practicing, be inspired by some lectures like
- Idiomatic Kotlin. Best Practices
- Best Practices for Unit Testing in Kotlin
- Take a look at some of Uberto's Kotlin Pearls
- Effective Kotlin
- Find something interesting to read here at https://kotlin.link/
- Be curious and discover something you did't know (still) in this awesome kotlin curated list: https://github.com/KotlinBy/awesome-kotlin
- See how the design patterns could be implemented in Kotlin https://github.com/dbacinski/Design-Patterns-In-Kotlin
- Kotlin Sequences: An Illustrated Guide
Moreover, if you really want to go deeper:
- Join the Kotlin Slack workspace
- Read and follow the Kotlin's official forum