You can learn the basics of combat from just about any nation. Just putz around with different troop types, see what gets you killed and what doesnt.
More important is learning how to use mages, which definie about 90% of the game's content. But that takes a lot of experimentation, so just have fun and putz around with them.
After that, I would read some of the excellent guides in the strategy section (
http://forum.shrapnelgames.com/showthread.php?t=38122) and on the wiki (
http://dom3.servegame.com/wiki/Nation_guide). Some of the strategies described are really elegant and complex, so dont worry about copying them to the letter. Just read them to get an idea of how to use the resources each nation has available. If you try out one or two of their tricks in each game you play, and you'll master them in no time.
Of particular note are two of Baalz excellent guides:
On Communions:
http://dom3.servegame.com/wiki/Nation_guide
and on Blood Hunting:
http://dom3.servegame.com/wiki/Reign...d_Bloodhunting
These were some of the harder things for me to figure out, and these guides helped tremendously.