There are four resources that you need to resupply (ore, food, stone and wood). There are resource buildings for each of them (lumber yard, farm, mine, and quarry). These buildings must be upgraded in order for them to store more of their corresponding resource. If a building reaches its storage capacity, it will no longer provide its resource.

As these storage reach their limit, you are not able to generate resources anymore.

When first starting, ignore your offense and defense and focus on your resources. Upgrade everything that affect the regeneration of your resources. You can collect resources frequently and easily when you are actively playing. You will eventually have to upgrade the storage capacity of your resource buildings so you can gather resources while not playing.

If you plan on logging out for an extended time, spend all your resources to prevent them from being stolen by other players stealing all your resources.