How do I get to be a farmer on last shelter?

Farmer is one of the classes. You choose you class first when you reach Base level 6 and build the class hall building. After this you can change you class for free when you reach level 10, 12, 16, 19, 22, and 25. OR you can buy a package with a class change item to do it at any other time. To change class just tap on the class building and chose the class option.