To recruit, you need to get to the recruitment. You do this by clicking on the "recruiting tent" icon on the city screen. Alternately, you can just double-click on a province on the main map. Once you've exposed the screen, you can see up/down arrows for each type of land unit. When you click the down arrow, one army unit's worth of the specific unit type is added to the number on the "Units Ordered" row; this is the proposed recruitment. Assuming you can afford the proposed recruitment, the "OK" button will be lit, and you can click it to initiate the recruitment operation. When you click OK, the recruitment screen will be closed, money spent, and an icon will appear on the map showing a blacksmith hammering away.