my advice is to pick one nation to play with for a while, experiment a bit, read the nation guides, play some more. Don't try a different nation each game, they play very differently
the early era nations are probably easiest, Niefel is fairly one-dimensional while still reasonably strong, especially in the early game
I played mostly Abysia when I started, as they are fairly simple. unfortunately they aren't very strong after the early game without planning to use blood magics extensively, which increases the complexity and micromanagement hugely. You may as well pick something decent
oh, also, note that it is a very mod heavy environment. The devs didn't even try to balance it, they left that to the community. The most common mod is CBM (Conceptual Balance Mod), which a lot of games are played under, certainly more tha Vanilla Dom 3. I'd recommend playing that mod from the start with your single player games, there is a big adjustment from vanilla to cbm and it's not worth the pain
disclaimer: i'm close to a year out of date, i doubt what I've said above has changed much but you never know