Use hills so you can run down from them and focus down single targets. Remember LOS rules for archers (UI could be better here). When you get horsemen use them to scout or sit back and swing in and finish things.
Barb spearmen like to chase you, so lure them out with slingers and then take the camp out with a scout.
Mostly these are basics. When you get later units you can do more interesting things, but the fundamentals are just knowing what has a bonus against what, and knowing the terrain effects.