When you select a castle town or a port, a recruitment tab will appear. Select it to see all units available for recruitment or ships that can be constructed there. Right-click on the unit or ship picture to view its description and statistics. Left-click on it to add it to the queue, and again to remove it. The cost of recruitment is displayed in the bottom left, the time required is shown in the bottom right of the picture. Ships and their crew are always recruited directly from a port building.