There are four types of resources that are used throughout the game to do just about everything. They are fuel, food, steel and alloy. The total that you have available, that you have gathered, is at the top center of the city screen.
When you begin the game, technology research, training troops, constructing buildings and adding resource plots uses only fuel and food. As the level of your city increases, all of these activities begin to require steel as well. Later in the upgrade process, alloy is also necessary.
These resources are obtained from plots. The primary plots are seen in the city screen, where you build them just outside the walls of the city. The mix of resources is up to you, limited only by the availability of spaces and the level that the city is at.
In the territory view, there are also resource plots available. These need to be farmed by your troops. This takes time. The speed is a function of the type and level of troops involved and the amount of resources in the plot. You direct the number and types of troops to send. When they have become fully loaded, they will return automatically to your city.
Partially farming a plot can be done but it is counterproductive in the long run. Not farming the entire amount of resource prevents new resource plots from spawning in that area. Cleaning up means more chances to obtain resources.
Your troops can be attack by other players while farming. When your alliance can build a resource building, it will permit protected plots to be created. Until then, farmer beware.