To produce more Resources, construct and Upgrade Townhouses, Mines and other Resource related buildings such as the Forge and the Dungeons. You can also boost your Gold and Iron production by constructing and upgrading the Altar of Weor and/or by benefitting from the bonuses that come with activated Paragon Status.
You need Food to maintain your Units. Every military Unit in the game consumes a certain amount of Food per hour. You will not be able to train new Units if you run out of Food. Food is collected from the Farms in your Castle. Build and upgrade your Farms along with the Spring of Life to increase the rate at which you can produce Food. You can construct a maximum of 6 Farms.
The amount of Food you can store at your Castle is limited by the capacity of your Barns, and the amount of Gold and Iron you can store at any given time is limited by the number of Warehouses and their capacity. You can construct a maximum of 3 Warehouses and 3 Barns. Make sure that you upgrade your Barns as you increase your Food production, or anything you produce over your maximum capacity will go to waste. Likewise, you would do well to ensure that you upgrade your Warehouses at the same rate as you improve your Gold and Iron production. For more information on possible ways to increase your Food production rate, consult the "How do I increase the rate of my Food acquisition?" article.
You can use Sapphires to temporarily boost your Resource production for 3 days by 25%. Go to the Black Market to get Resource Production Boosts or packs of Resources.