Basically what you have to do is plant crops and harvest them.

To plant crops you need to prepare the land (you click on the little agricultural tool), and now it's ready to have something planted on it. You click on the store icon and you can buy seeds and then plant them. Later you come back to harvest them.

That's the basic premise. You can decorate your farm with fences, buildings, trees, animals, etc. I don't really get the appeal of the game but I know some people love it. I tried playing it once and that was it, so I'm sure someone can explain it better.