Click on one of the factory buildings to bring up a factory production dialog. In this box you see a production equation explaining what this factory can do.
To order production at a factory click on the arrow to the right of the slider until the maximum amount of production (capacity) is reached, or until you run out of a necessary item. You can change your mind and reduce or eliminate production by clicking on the other arrow.
All materials have multiple uses, so it is not always the right decision to produce all the goods you can.
