Don't know if you'd still like an answer, but maybe this is of use for someone:
To build anything you first need at least one extractor of clay, wood or quarystone. (Since you need those building materials a lot, and also the building materials that can be produced by a potter and a stonemason - professions for experienced extractors of clay and quarystone - it's usualy wise to asign an extractor for both clay, wood and quarystone right away.) Now, if you turn at least one civilian into a builder, you can build a farm. As soon as a farm is build and a farmer is working in it, you can build some more, like a dwelling 1, or a defence tower1. Once your extractors and other workers gain enough experience, you'll be able to build other buildings or upgrade buidings.