If you have free peasants in your village who aren't working at one of the village buildings, you can turn them into troops by clicking on the army icon. You need to have researched the troop type first and some troop types such as an archer will require that you have made the needed weapon (bow) to recruit the unit. The overall size of your army is governed by the ‘Command’ research, which increases the number of units in your army. Note ‘units’ are also used when you add scouts, traders and monks to your village. Over time you will adjust this balance to suit your play style.