Click on your Barracks, you will see diffferent types of troops available, i.e. Supply, Militia, Scouts, Swordsman, Archers, Cavalry, Heavy Cavalry. Click on the one you want to 'train'. If you see red highlighted areas in the popup box, after clicking, you need to first obtain what it is telling you. Example, if Level 5 Barracks is highlighted in red, you need to first upgrade your barracks to a level 5 before it will let you train that particular troop. If nothing is highlighted in red, enter in the amount that you want to train and it will show you the time it takes for training to be be completed.