In order to produce goods for trade, you must create an industry for that good.
Industries usually consist of raw materials and a finished product.
For example, let's say that a neighboring city is interested in buying your surplus pottery and they are willing to pay handsomely for it. You will first need to collect clay at a Clay Pit. Once the clay has been collected, you can then have the raw material finished into sellable pottery at a Potter's Hut. This combination of raw materials and finished goods is common through out the game. Determining what you can produce Not every item that is in demand can be produced by your city.