You start off with two Priests, a Granary and a Storage Pit. There's two Mitanni towns on the map: a yellow one, just north of your starting position, and a red one across the water (both are Assyrians). Move north, and start converting villagers. Try cornering them one at a time - if they gang up and attack, your Priests could die. When you have converted all of their Villagers, and you're sure they're not training any more, start building up your town, training more Villagers.