One of the great things about this game is that people do not tend to agree on most of it. Everyone has favorite nations. They all play differently so you will want to try them all. The "best" nation will be the one that best matches your playing style. Slow moving heavy armor? fast moving raiders? heavy research? barbarian horde? stealth guerilla warfare? undead armies?
Personally for new players I like to recommend Middle Age Man. Its not the most fun, nor the easiest to win with. But its probably the most "standard" nation. Its bowmen and knights, mage, priests, scouts. Not very hard to figure out what each one basically does so you can concentrate on learning the menus and keypresses.
There is a map on
www.Dom3Minions.com called Tower which is good for new players. It allows you to play with many AIs without getting swarmed by them. It also tends to improve the use of some menus (such as F1)
Also a good tip for new players is that the Independents strength in games is the reverse of AI strength. If the game is rough on you, turn the indepts UP. Most people turn them down thinking it makes the game easier but it actually makes the AIs harder.
If you get tired of Solo in vanilla mode then there are maps and mods to boost it.
Oh yeah, and the only almost complete keypress chart is at the player-made wiki.
http://dom3.servegame.com/wiki/Hotkey