I suggest that you go to the options, and select the workers to leave the trees, and to leave existing improvements. Otherwise they will chop everything down and build farms everywhere. I think it is best to control them manually at first, so that they do exactly what you want. Priorities are to improve special resources, and build roads to them. Second priority is cottages and farms, or tree chopping if you want to hurry a building. I switch them to automated once I have the land round my first few cities improved, then later if I see something that I want done I take some back under manual control and send them to do it. Times when this might happen are when a new building becomes available (workshop, waterwheel, windmill, lumbermill), or when a new tech changes the output from an improvement (e.g. when windmills get +2 gold I might decide to change some mines to windmills). When a city has the possibility to build a specialist I might decide to add a farm so that I can have that specialist.
Having said the above, when new to the game, I found it quite instructive to watch the priorities that the automated workers have.