Farmer
Farmer is a vital profession for the survival of one's fortress, as there's not a better profession for obtaining food. To hire a farmer you'll need to build a forester's hut. Farmers are better then foresters at gathering food as you need much less farmers to sustain a colony.
Use
Farmers will work on any building in the "Farms" building tab. Or you can make a farming zone for free, but you'll still need to build a farm first.