Good ol' Rome: Total War, I think I will explain the easiest strategy in the world to you its unbelievably simple and works well whilst you figure out your own ways to position your army and eventually gain your own tactics. Well on to the Strat:
1. The simple "Double Layer" technique, all you do is heavy infantry (Histatia, Pretorians etc) in the front line, with a good back line of Missile units (archers etc).
2. Cavalry flanks, with the "Double Layer" tech it's always good to have Cavalry on your flanks, both light and heavy do the trick.
That is pretty much the easiest effective way to set up your army.
A few things to remember:
1. NEVER! charge cavalry head on into spearmen.
2. When defending a siege use archers to target enemies with siege equipment e.g Ladders, siege towers etc
3. Easiest way to take out a Greek/Macedonian Spear phalanx is to hit it from the side or behind with heavy infantry, also Cavalry in the rear will work and you should take to many casualties.
4. Use cavalry to pick of fleeing enemies, LEAVE NOONE ALIVE! unless its rebels no matter what they loose all their army if beaten.
5. The counter charge, never let your men just stand there and take a charge, they will be annihilated.(remember to time this perfect so your lines don't break)
6. Archers are your best friend, use them wisely target threats not peasants, your archers have a limited supply of ammo and you want them to weaken strong units.
7. Cavalry > Archers, or in other words hit enemy archers with any free cavalry units.
8. Cavalry charge the side or rear of enemy units, when possible, this will not only break enemy lines, but destroy there morale.(never get your cavalry trapped, only hit from rear if behind the unit is clear space, reinforcement charge on the rear of cavalry will end badly for you)
9. Use spearmen for enemy cavalry.(only exception for letting your soldiers be charged is cavalry into spearmen cos spearmen will win most of the time)
Well thats the basics for you, you'll work out your own tactics in time.
Good luck General!