Storage Units cost 20 [wood] per unit of storage (which is four land squares), from one unit to an infinite number of them. They keep one worker busy while under construction, then provide no jobs once opened.
To Create a Storage Yard Select the Hammer icon at the bottom of the screen and select Storage Yard. Click-and-drag to outline the Storage Yard footprint and your citizens will add its construction into their list of projects.
Storage Yards fill up fast in the early game. Later, you will replace them with organized warehouses, which hold much denser amounts of goods in the same terrain.