Modding is one of Factorios greatest features.
Since v0.11 all (new) mods needs to have a license. If a mod has now license it is assumed to have the a creative commons license: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0).
See also this forum article.
Creating a scenario from a saved game (map)
./bin/prod/factorio --map2scenario mapfoo
Creating scenarios with the map-editor
Another first step is also making maps with the ingame editor. That stores the map as a scenario, which is just a directory on disk with some files in it. You can look into that files and try to understand, what they are for. Or you can read the following pages: