If you have a Windows PC, I can strongly recommend Microsoft 3D Builder.
It has a very low-slope learning curve and lets you make or modify STL files very easily.
- Supports .3MF, .OBJ, .WRL, .STL, .PLY, .GLB, and .GLTF.
- Imports image files and converts them into models. (eg. for lithophanes)
- Allows Joining, Merging, Cutting, Splitting, Scaling, Extruding, Embossing, etc…
3D Builder allowed me to start my printing hobby by downloading and cutting/pasting other people’s models, until I could learn to make my own.
It is also integrated with Netfabb in the background, so it is able to recognize and often can repair non-manifold models.