Instructions
Click on one of your "Explorers." These types of units are likely wandering your existing civilization and will be either building new structures or mining for resources.
Click the "Town Center" button in the toolbar for your explorer. Every town center you build removes the existing population limit, allowing you to build more structures and add more troops and other units to your civilization.
Click inside your civilization at the exact point where you want your town center to go. Your explorer will immediately begin to build the town center. When it is complete, your population limit will increase.