Creating a terrain will add a flat-level surface on which to build the world details.

In the Hierarchy panel:

Click the Create option

option Scroll down to 3D Objects

Select Terrain

With the Terrain created, it's best to zoom out using the mouse and get accustomed to navigating the view of the surface. Also, in the top right corner of the Screen panel is a set of Red/Green/Blue arrows which change the view to the X, Y, or Z axis when selected.

Another set of Red/Green/Blue (center of the Scene panel) show that the element Terrain is selected (which can also been seen highlighted in the Hierarchy panel); by clicking and dragging these arrows, the Terrain can be manipulated within the world along the X, Y, or Z axis.