There are four resources available: crop, wood, gold, and iron. This page outlines the different ways these resources are gathered. It is worth noting that attacking other players is also a good way of obtaining resources.
Crop is gathered primarily through Farms. A Farm will produce one crop per hour for each laborer working on the Farm, and will also have percentage production increases starting at level 7 and 10%. A small amount of Crop is also gathered by the Town Center; the amount depends on the level of the Town Center. Gold may also be traded for crop at a Market of level 3 or higher, at the rate of 50 gold for 100 crop.
Crop is needed in the development of most buildings and units early on in the game, and you'll need crop to sustain an army. The bigger your army, the more crop income you'll need to sustain it.
Wood is gathered primarily through Lumbermills and the Lumbering Camps created by them. A Lumbering Camp will produce wood according to the level of the forest, the number of laborers assigned and the number of other Lumbering Camps already at the forest. Wood is also gathered by the Town Center; the amount depends on the level of the Town Center. Gold may also be traded for wood at a Market of level 3 or higher.
Wood is needed in the development of most buildings and units early on in the game.
Gold is gathered primarily through Markets. Gold is earned at a Market by selling crop, wood or iron. Additionally, Markets of level 4 and higher will passively produce gold at a rate dependent on the level of the Market, starting a 4 Gold per hour. Gold is also produced by Workshops of level 4 or higher if Weapon Export is researched at that Workshop. The rate of production is dependent on the level to which Weapon Export is researched.
Gold is needed later in the game, to make mid-level and beyond units and buildings. It is also very useful for trading for other resources when you are in need of them and have an excess of unwanted gold.
Gold is very useful as a storage resource, as it has no cap and there is no downside to have an excess of gold. While other resources may be easier to get, with the exception of iron which is harder, they are limited by a maximum amount you can have in any city at any one time.
Iron is gathered primarily through Refineries. The amount of iron produced by a Refinery depends on the level of the refinery and the number of laborers assigned. Gold may also be traded for iron at a Market of level 8 or higher.
Iron is needed later in the game, to make higher-level buildings and units.