Well....
First thing to do is decide what each army has. You need to take in account the type of land the battle is going to be on. Open plains, rolling hills, and other wide open spaces plays into the hands of Calvery and archers. On the other hand, mountain passes and other confined spaces are more for massed infantry.
Now, there are different types of Infantry and Calvery. Basiclly, you have Light, medium and Heavy units. Light units are scouts, harassing the enemy and slowing him up. They cannot stand up to an all out attack by heavier units. They can move fast and can bed all overt the battlefield. Your scouts are light infantry.
Medium units are a combination of speed and firepower. Slower the light units, they have hevier armor and weapons then light units. Heavy units are usually the elite, the career soldiers or nobility. They are slower then the other units, but they can ground up the others.
There is a series of PC games called Total War. Right now, there is TW: Shogun, TW Midive, and TW: Rome. The key part of the game is the battles, as you army goes up against other armies. You have to chose what type of units to put in your army (Depending on your location and tech level).
Go to this web site:
http://www.totalwar.org/strategy/twug/index.html And it has a description of all the different military units in the different games. It'll give you an idea of the different units you can incorprate into your story.
Craig