IntelliJ IDEA comes in 2 versions. The Ultimate and the Community Version. The community version is an open source version and licensed under the Apache 2.0. The ultimate (payed) versio is for enterprise development and web development.
You can download IntelliJ IDEA from the JetBrains Website.
Which Version to download?
Even the community version comes with a lot of features. Especially for the Kotlin development, this version can be recommended. Additionally this version supports Android development, version control (Git), deployment functionality (Docker), build tools (Maven, Gradle) and of course standard equipment like a Debugger, Testing etc.
For an extensive list of differences of both version checkout the following link.
This tutorial will use the community version of IntelliJ IDEA.
Download your version
You can download the community or ultimate version from the link posted earlier. For the Windows version you will get an executable which will be located in your Download folder.