You can easily start coding in Kotlin online with the official Kotlin Playground available here. This will help you to write some basic programs in Kotlin. After learning basic things you can switch to offline IDE (which will work without internet).
Getting started with Kotlin is easy using the Kotlin plugin integrated into IntelliJ IDEA. Benefit from code highlighting, code completion, refactoring, debugging, and other features available for Kotlin.
If you want to start from scratch, you can build a basic Kotlin application. Once you create the application, you can dive deep into the Kotlin syntax:
Add sample code from Kotlin examples
Install EduTools plugin for IDEA and complete practice with Kotlin const course
If you already have a project build in Java, you can:
Start adding Kotlin code to your project.
Start with a small class or function with the required annotations for proper spacing with Java code.
Write some tests in Kotlin for your code.
Tests are a safe place to add Kotlin to the codebase as they are not bundled with your application during packaging.
Convert Java code to Kotlin.
Remove small bits of Java functionality and work using Kotlin classes and Java-to-Kotlin converter.