Many of the Starter Worlds have a section of code called a "tweaker's box" at the top. This contains the easiest parts of code to change.
For example, Blank Three.js World is quite simple to edit. It has multiple sphere objects, stored in a structure called an Array.
The following are some Three.js Starter Worlds. When you get more advanced, you will make your own entire Worlds. The Worlds you create are only limited by your imagination.