TinkerCAD

In short: TinkerCAD is a free, browser-based 3D program by Autodesk, especially for beginners.

In more detail: You combine simple basic shapes or cut holes. The result can be exported as STL for the 3D printer.

In Depth

Strengths

Very easy to learn, runs without installation, many templates. Electronics simulation and block programming are also included.

Limits

For complex, parametric design professionals use CAD such as Fusion 360. See Blender.

See also: CAD, Fusion 360, 3D Printers, Blender