Free tools for learning 3D primitives include Blender (open-source, with pre-built shapes and beginner tutorials), Tinkercad (web-based, drag-and-drop for basic forms), and SketchUp Free (intuitive for drawing/extruding primitives). These let beginners practice manipulating shapes—resizing, merging, cutting—to build foundational skills at no cost.
For new learners, start with Tinkercad for its simplicity, or Blender if you want to explore advanced primitive modeling later.