To get started with development, install IntelliJ IDEA (it's free!!).
Once IntelliJ is installed, open the project directory. If you have not already downloaded it (e.g. using GitHub Desktop), you can use the welcome screen to clone the GitHub repo directly by selecting "Get from VCS" and then logging in with your GitHub account.
Once the project has opened, you will need to wait until IntelliJ finishes importing the project and downloading dependencies. Depending on your computer this may take a few minutes.
After the project has imported, go to File > Settings, then change the following settings to "IntelliJ IDEA":
You should then be able to run the game by selecting "Run game" in the top-right, and then pressing play.
If assets (especially registry objects) are failing to load, make sure you have run the generateAssetList Gradle task (this should automatically be run when running the game in IntelliJ)
To build a release of the game, run the clean ``build task in Gradle (eg. ./gradlew clean build on the command line), and the output JAR file will be in lwjgl3/build/libs/.
You can also automatically generate a list of libraries and their licenses using the downloadLicenses Gradle task, which will create a report in lwjgl3/build/reports/license.