The game allows you to manually drive the machines or you can hire workers so they can do certain tasks for you. If you want to develop your farm you are forced to hire workers or other players in the multiplayer mode. You can hire as many workers as you want, however, you have to pay them (1$/second per worker). You can also order them to drive your machines to an appointed place on the map or to complete a certain task (e.g. selling animals or filling a fertilizer vehicle).
If you want to hire a worker you only have to place any of your machines next to a field and press the steering wheel icon. You should place your machine on the edge of a field so a worker won't miss anything. However, workers can't handle some of the obstacles (trees, other machines on their way) so you should check their work from time to time and when there is a problem you must manually correct a problem and call a worker again. They can refill your machines by themselves, e.g. vehicles for fertilizing or planting as well as refilling fuel.
In the case of tippers and vehicles with animals - when you press the steering wheel icon you get access to an additional menu in which you can choose a destination. If the machine is a combine you can also use horn icon - using it when a combine is full calls a nearby vehicle with a tipper (it must be next to the same field) which allows a combine to empty its content.
If you want to give other orders you must open the map. After pressing the steering wheel icon in the bottom right corner of the screen you can select any place on the map, it can be a building or a machine, and the computer will try to deliver that vehicle in the appointed place. Other icons allows you to refill machines, fill up the petrol tank in your vehicles or go to a car wash (it is only a visual element, it has no impact on machine's performance).