1. Open a new 3D Scene
2. GameObject > 3D Object > Terrain
3. Zoom out so you can see your entire Terrain.
Use the scrolling option on your mouse and the hand tool on the top of unity to get into a good view.
4. Click on terrain with the MOVE tool:
6. Customize your Terrain.
7. Open the Asset Store.
8. Download the "Standard Assets."
9. Click open in unity. Install with the arrow in your "Package Manager."
10. Choose Paint Texture
11. Go to EDIT TERRAIN LAYERS find a flat surface to use as your base. Add other terrain layers and use your paint brush with that layer selected to add the textures.
12. Add Trees.
Click the tree icon. Go to Edit Trees and add tree to your menu. Place on scene with mouse.
11. Add Water.
Assets> Standard Assets> Environment > Water > Water4> Prefabs
12. Move your camera.
Use the move tool and select the Main Camera. Look at the preview to see.