The 5 resources in your game are Food, Wood, Stone, Metals and Gold.
You get food from building farms in the field. Food is the lifeblood of the city. Food is used to feed Troops and Dragons.
More farms = more food production = larger army you can maintain.
If your troops are eating more food than you produce (minus food production), you will not lose any troops from "starvation". Your food reserves will stay at 0 until you fill up your reserves by raiding or building/upgrading farms.
You can attack Antro Camps to get lots of Food. Although, you'd have to hit it constantly to maintain a good army, attacking a high level Camp could give you well over half a million in Food.
You get Lumber from building lumbermills in the field.
- Lumber is the resource you need the most for buildings and troop training. There is nothing you can build or train that doesn't require Lumber. It is also used to Research Woodcraft, Weapons Calibration, Metallurgy and Levitation.
You get Metal from building mines in the field.
- Metal is required for all but 3 types of research. With so many uses you may find yourself running low on Metals even though the individual upgrades, research, and troops you are working on don't seem to need much of it. So stay on top of Metal production if you don't want to be left behind.
If you keep an eye on your production and usage you shouldn't have much problem keeping up with demand for Metals. Building 4 or 5 Mines early will help boost your growth and still be able to keep up with research needs. Later on when you start to build a larger army with stronger units metal is used as much, if not more than wood, and is easily your most precious resource...
You get Stone from building quarries in the field.
- It is extremely valuable during the initial stages of building up your Fortress, Science Center and Garrisons. But after that, its usefulness quickly diminishes. For early development, upgrading only a few Quarries is just as good as building new ones. Even though upgrading one takes longer than building new ones it is more benefitial that way because you can build more of the other 4 types of field buildings.
- Stone is used mainly for buildings and even then it is only the main resource in your Fortress, Garrisons, Sentinel, Muster Point and Wall. All others require more lumber than anything else.
Gold is earned from citizens of your city paying taxes. more Homes=more citizens=more Gold. Gold is used a lot for technology and you may be running low because a lot of your spaces are used to upgrade Garrisons and less for Homes. You can also get Gold by selling resources in Trade.
Rubies are the special in game currency that is used to purchase different things to further enhance the game in the favor of the player. Rubies may be purchased or earned by completing a variety of your "free offers"(even the free ones would require personal information and some money).