The most fundamental unit of the Farming Set is the "Seed (Tier 1 Farm Item)".
You can get any Tier 1 items (not just for Farming Set) by playing the game (match 4+ items in any of settlements, or open treasure chests) or by buying them from the in-game store. Once you're further along in the Capital City, your higher tier buildings (such as "Crop") will also generate Tier 1 items (such as "Seed").
When you combine three (or more) seeds, you receive a "Crop (Tier 2 Farm Item)":
Combining three (or more) crops will result in creating a "Grain (Tier 3 Farm Item)":
Combining three (or more) grains will create a next tier building - a "Grocer", and so on.
Here is the progression list of buildings in the Farm set:
The rewards you receive from each set are based on the highest building you have completed (so if you have Grain, Grocer and several Seeds, your rewards will be calculated based on the Grocer because it's the highest level building), and they scale up accordingly.
Farming Set will produce basic game rewards such as Grasses, Bushes... and if you create a Hotel (highest level Farming Set building), you have a chance for a House to drop!