Adrea,

At the top left of your screen when you are in Town, is a resource counter. It shows you your available amount of Gold (top left), Lumber (top right), Character bound Shardium (bottom left), and Account Shardium (bottom right). If either the gold amount or lumber amounts are RED numbers, you have hit the max amount you can currently hold.

You'll want to spend some lumber and level up those buildings! The amounts of lumber and gold you can hold are increased by leveling up your Warehouse. =^.^=