Modding is relatively easy. I you wan't to edit say... the stats of an unit or the name of a nation you will do fine with wordpad or something similar. If you wan't to make an entire nation you'll do fine with a text editor and a program like Paint(Windows) to make unit graphics. Be aware of the fact that not everything can be modded in Dom3. It's easy to do it but not 100% modable. You just can't have it all!
Read the modding manual that comes with the game. It may seem a little overwhelming at first, but you'll learn.
On the subject of the most newbie friendly nation I would you say that Tien Chi is a good nation to start with. Strong troops that are not to expensive and good summons.