You might take a glance at TinyNation.dm on my site
http://www.dom3minions.com/mods.htm
which isnt very playable but it does create a nation and tries to explain each command. It should at least convince you that creating a nation is easy.
Then grab a more complete nation (Sombre's might be the most extensive unless someone suggests another). It might be daunting t grab his right off the bat.
The modding manual is in your games /doc/ directory. Something like this?
file://C:\Program Files\dominions3\doc\modding.pdf
If you make new units there are two sprites. Basically one standing, and one when it does its attack. Usually all you need to work on is the standing one. The attacking one can be a cut-n-paste where you move one arm, or add some glowing dots above its head. They need to be .tga files so you might need a program to convert the images after you edit them if your paint program doesnt allow "save as" in tga. I recommend always saving in the native format of the paint program, then saving one as tga since the tga copy can lose some data that makes it easier to make changes later.
One of the hardest things seems to be packaging the final product. Many modders first efforts seem to have trouble creating a compressed file that unpacks in a standard structure. IMHO it is best to create a file which can be copied to the /mods/ directory then unzipped to create a single .dm file and a single directory named after the mod with all the other files in it. Some end up in multiple subdirectories or even no directory at all which is confusing to new players.