There are 3 types of warriors:

Spearmen costs 3 coins

Archers costs 10 coins

Knights costs 30 coins

At the bottom of the screen is a Buy Warrior Panel. Tap at a warrior to select. If you have enough coins it will be sent for training and then into the battlefield. Every 15 seconds you earn bonus coins. Every kill earns you either 3, 10 or 30 coins depending on which enemy is killed.