Modding is one of Factorios greatest features.
A good overview is the list of mods or the Factorio forum.
A good starting point is also the modding-Help-Board. The community is always answering to concrete questions. Sometimes within minutes. Or you could try the Modding FAQ page.
An often asked question is how to make a scenario (something which can be edited in the map-editor) from a map:
Factorio can convert them (see also this post):
./bin/prod/factorio --map2scenario mapfoo
More about the client parameters
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: