You can earn coins by going to your neighbors and tending their farms, and going to the market and try and get hired to work on someone else's farm by harvesting their crops. Also, when you are on the farm town application, in the top corner there are some options like Play, Send, Neighbors, etc, and there is one in red that says "earn coins." You can try doing some of the things that they have until that button if you click on it.