You just go to the city status screen, and click on the terrain (like a hill, whale, fish, whatever) to put the citizen to work on it. Click again to take him off.
Take a look at your resources as you shift citizens around to different tiles to work and/or use taxmen/scientists.
A fast way to put eveyone to work with one click.... click on the map center (your city). The computer will re-arrange your citizens for "most food"... which is usually not the absolute best mix at any given time.