There are 3 types of soldier:

Rifle marines costs 3 coins

Snipers costs 10 coins

Rocket commando costs 30 coins

At the bottom of the screen is a Buy Soldier Panel.Tap at a soldier 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.