Well, my suggestion is keep a few number of faction just to start. Dont go in 20 faction, and 200 units, keep it low profile, can add later :)
For the map, i suggest you ask to people like BDH who is expert mapper, and will give you a starting map, i hope, to work on.
Send a pm to him asking help :)
Other thing, you must decide how many units for faction, and every faction provide a very detailed description, or even picture if find some.
With that yo ucan start making units.
And then must script the campaign, and all :)
I can help sometimes makeing some units, but dont count on me too much :)
Bye