I don’t know a bunch of stuff, let’s find out what, and remedy the situation.
I opened a new 3D project and stared at the screen. What’s the first step? We need a character, and something for it to run on.
Googled: “Add character 3D world”, clicked first link.
Terrain tool I’ve never used this.
Creating a new terrain: GameObject menu -> 3D Object -> Terrain
Adjusting terrain contour Click the icon that looks like a paint brush in the terrain Inspector.