In this case, use the villagers you already have by taking them off other tasks. For instance, in AoE 3, a villager usually costs 100 Food to create.
Click this button as many times as the number of villagers you want to create.
A panel will be displayed in the bottom-left or right corner of the screen containing buttons for all the functions that the Town Center can perform. Villagers (or Settlers, depending on which version of AoE you are playing) are required to gather Wood for constructing houses, and they are also the units that do the actual construction of buildings.
