You need a ground of ares to train units. You have one from the start, but you can build more for 340 coins and 20 population. (tools-> shop -> military) The fastest way to do this is build slingers (90 coins). They will take 10 seconds to train, so with 2 or 3 ground of ares, you should get this within 5 minutes.
You can see your progress if you press Tools-> Achievements.