You can train new units in each settlement that is under your control. Training units costs Gold and draws on the population of the settlement. You cannot train new units in settlements that have no population. If the settlement has insufficient population to train a full unit, you can train a smaller unit equal to the remaining population. The population is not removed from the settlement until the unit has finished training.
Each race can only train one type of unit from all its settlements.
In order to raise different types of units, you need to Deploy Heroes.