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.
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: