Perhaps the most infuriating part of Hay Day is the way players are curtailed by the limited size of Barns and Silos. It is strongly recommended that you save your Diamonds for purchasing the raw materials needed to increase the size of the Barn and Silo, as some of the items can be very scarce to come by.
Tap the Barn/Silo and then click on the increase storage option. You will then see all of the items that you need to be able to upgrade. Nails, Wood, Planks and so on. These are found whenever you collect an item, harvest a crop, or check a mystery crate. The chances of finding them are slim, but you may be able to find them for sale from another player if you get lucky.
It won't be long before you have to increase your Silo and Barn storage