Managing the resources of a city is important for the successful development of the city. There must always be enough Food in the reserves for feeding the population and the armies hosted in the city. Also there must be enough resources to pay for the upkeep of the city buildings.
As the population and army size increases so does the Food consumption. You can check the Food production/consumption in the main tab of the city. There are two ways to increase the Food production:
1. Upgrading Farming
2. Upgrading Hunting/Fishing Guild
Choosing which one to focus for upgrading depends on the attributes of the city, if the Farming attribute is higher than the Hunting/Fishing (you can check that in the Main tab of the city), upgrade Farming, else upgrade Hunting/Fishing Guild. The Farming and Hunting/Fishing attributes of a city depend on the terrain that surrounds the city, if the city is near a forest or surrounded by water then the Hunting/Fishing attribute will be higher. If the city is surrounded by fields and plains the Farming attribute will be higher.
Buildings have an upkeep cost for maintaining, the bigger the building is, so is the upkeep cost. The upkeep is paid in Lumber, Stone, Iron and Gold. If the city fails to pay for its upkeep cost then the buildings will start to decay. You can check the resources production/consumption in the main tab of the city.
Each resource production building (ex. Lumberjack Guild, Stone Quarry ...) has a minimum population requirement to work at 100% productivity. If the population is below the limit then the production of the goods will be lower. The productivity of each resource production building can be checked in the city main tab.