How do become a farmer now I'm in the game?

You get to choose a class when you build the Class Hall Building which opens up after you reach base level 6. You can choose to be 1 of 4 classes. If you have already picked a different class to farmer then you get the option to change class for free when you reach base level 10, 13, 16, 19, 22, and 25. You can change at other times as well but you have to buy a class change item to do it.