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...
There are several ways: buy using a credit card; upgrade your level; do tasks in your workbay till you get 5 stars; and join special events or do special tasks.
Go to the map in the bottom right hand corner and go to the Retail Office. It will automatically pop up a screen and you can choose to upgrade your farm.