Use the right units. For example in the quest “We Killed This Guys Already” the best units to use in the beginning are Archers and Hypaspists, then later on Javelin Thrower and Hoplites. The reason for this is because the AI is mass producing horse units by the time they reach Iron Age.
Use the terrain, take note where the enemies could attack and seal the entrances. One example is the quest “You Complete Me” where there is only one entrance and you seal that entrance till you have enough army to attack.
Scout is the best unit ever! You can use scout to delay the enemy units from attacking your city since the enemy will chase your scout until it is dead. Just make sure you don’t get to far away from the enemy or they will ignore your scout and head for your Town Centre. To do this just get your scout near the enemy unit till they start chasing it, and then just run around in circles.
Do not ever build spearman when not needed! Because it is a waste of gold and food for the starting age. Only produce it if you have good equipment and your enemy is mass producing horse at the very beginning, until you reach Golden Age.
Cows! Who don’t love cows? They been our source of food since the very beginning so start searching for cows! Cows can be very useful if you run out of berries and still cutting woods.
Start building Walls! Walls keep you safe from danger. It is the best to build wall at the very start to keep you safe from enemy attacks. Not only you are delaying your enemy from attacking you, but you are also keeping your villagers safe while you dig for resources.
That is all for now. I let you figure out the others so that you could learn a little bit by your self.