Well, skinning is maybe not the best part to start off.
The easiest possible thing is maybe to edit army starting positions and compositions in data/world/maps/campaign/imperial_campaign/descr_strat.txt or change a few buildings that settlements have.

I know there's no real "how to start modding" guide or anything, but I guess we all kind of assume that people will have a look at the game files themselves before trying to learn anything through tutorials.