STLs are stereolithography files — 3D triangular meshes which represent 3D shapes — they’re not really editable (though you can load them into for example Microsoft 3D Builder and modify them to some extent).
You’ll need a 3rd party 3D CAM tool which will import STLs to make them into G-Code — options include:
- MeshCAM from http://www.grzsoftware.com/
- Freemill
- pyCAM
The former is the easiest, but is an additional purchase.
I would suggest starting with “Design and Make” — there are a number of free 3D models and that will at least get you started. I did a basic tutorial on this at: