How do I create a place for my characters to stand?

Scene panel -> Create -> Plane

Scale the plane to 100, 1, 100 and set it's position to 0,0,0. Click on the plane in the Scene view to active it's inspector panel on the right. Add a component to the plane, physics -> mesh collider. Then when you Instantiate your character, make sure he is above 0,1,0, like 0,2,0 so he doesn't fall through, and then he'll collide with the plane and stand under gravity.