While planting various crops in empty plots, you will need to use your Seeds.

Each crop costs a certain amount of Seeds. You can check that out by tapping and holding onto the crop to see the Seeds cost. You can get more Seeds by clearing out Grass. The bigger the Grass, the more Seeds you get!

When you have enough, tap and drag your desired crop to empty plots then wait for them to grow. If you want to speed things up, tap on a plot and tap on the Fertilizer!
Once the crops are ready, tap on a plot and drag the sickle to harvest.