Create a fork on your github account, then git download your fork, eg:
$ Git clone https://github.com/SpaghettiDeveloper/Terasology.git
Enters Terasology folder, open the terminal and enter the command:
$ ./gradlew Eclipse
or
$ ./gradlew idea
Open Eclipse, IntelliJ (I recommend IntelliJ), and your project will configure itself when opened!
Terasology use Gradle, so if you create your form (you have to create you), I suggest you give the command:
$ ./gradlew
in this way you create the build of your form with all the tree of folders.
Go on the wiki, and more, if you've never used Gradle or github, you have a lot to study my friend
Official Wiki:
https://github.com/MovingBlocks/Terasology/wiki
Use it, it is precious!