First of all, and this is very important before you start the game, the manual informs you of the relative difficulty of each nation.  Since nobody here has the manual, I wanted to share that so hopefully people won't start off in hard mode and decide the game is terrible.
Easy - Caerlon
Medium - New Almekia, Leonia
Hard - Norgard, Iscalio

After you start a game, one of the first things you'll probably want to do is look at your units.  For the most part the stats are nothing unusual, but a few may be confusing at first.  For one thing, the little colored dots underneath the unit.  These are the unit's element, and everything will have one to three dots.  Red opposes blue, green opposes yellow and white opposes black.  If I recall correctly, a unit with red and blue just has them cancel each other out.  If a unit with two red attacks a unit with blue, the bonus to damage dealt and penalty to damage taken is doubled - if both units have blue but no red, they deal reduced damage to each other.

Your leader units (the humans) have two stats that will be extremely important to you:  Rune Power and Rune Area.  Rune Power is the total value of monstrous units they can command, each unit will have its own cost and those costs added together cannot be greater than the commander's rune power.  Rune area is the hexagonal radius inside which the troops under their command get stat bonuses.

Another important point to remember is that all units have attacks with varying effectiveness against sky and ground targets.  If you look at an attack and see a circle, then it's at 100% effect against the stated target.  A triangle means... 80%, I think it was.  Nothing new but something to bear in mind anyway, not all attacks have the same range - keep an eye on how far away an attack will hit before you move a unit into position.  And if there's an 'A' next to the attack, it will hit everything within that range. A 'B' means that it will hit a line between the user and the range specified.

Some monsters will look worthless.  Don't be fooled, some of those become useful as they advance.  Remember also that everyone on the surviving side of a fight gains 200 experience, even if they didn't do anything.  All units can advance once they hit level 10, and most can advance again at 20, sometimes requiring an item to do so.  You can always recruit new ones (if you have the mana) to replace losses by choosing 'summon' at a city.  Different cities have different summons available, even within a nation's starting boundaries.

A final point that I think is important to make for new players who may otherwise have trouble - questing is a very good idea.  You can get new leaders, new units, new abilities for the leader that went questing... you can also get them injured, but if so they'll just have to rest for a turn or two and then will be usable again.  So don't leave all of your leaders sitting on your borders waiting for attack (and definitely don't leave them sitting elsewhere), send those you don't think you need right now out into the world.

Quick edit because I forgot something important.  If a leader gets killed in battle, they are sent, badly injured, back to the city to recover.  But since the leader isn't there anymore, all of their units will either leave or sit around being confused and aimless, and will not fight except to counter attack.  These abandoned monsters can then be captured and used in your own army if you win the fight.  If all enemy leaders are removed from the battlefield, the fight is over.