Here is a free course from the Decentraland University VR Academy, on Building Virtual Reality Scenes in Decentraland
https://docs.google.com/document/d/1AF9l3lYzQsiwFcIexGKP3TI-FFUcMVE34Su6U7FuhZo/
It introduces all the basic skills you need to become a capable builder, from the beginning. This document contains the course materials, links to video recordings of the live class sessions from the inaugural offering of the course, the assignments, and a link to the Discord server of the Decentraland University, in which the VR Academy channels support interaction between the students, alumni, and instructor.