Training your units costs Food. So check your Stats to see how much Food you have. For every additional unit, you will have to pay 10 units of food. So if you want to add 2 warriors it will cost 2x 10 units of food. Workers create income, your Gold. Farmers create Food. So you need all 4 types of units.

By choosing to Un-Train your units you will get Food back. For every unit you un-train, you will get 8 units of food back. Then you can use that food to train other units, so for example you can reduce your workers and get more more warriors.