Create a Plane GameObject using the Hierarchy window create menu:

Create a Sphere GameObject using the Hierarchy window create menu:

Both game objects should be now positioned in the center of the 3D space (0, 0, 0):

You can reset an object position using the Transform component's "gear" menu in the Inspector window:

Select the Sphere game object in the Hierarchy window and lift it up by setting the Y Position of its Transform component to 0.5:

Let's color the ball so it's easier to see it.

Create a new folder called Materials in the Assets folder. Create a new material called Pink inside it (using the Project window Create menu):

Select the Pink material in the Project window and set its color in the Inspector window. I'm using the hexadecimal value D61594:

Drag the Pink material on the Sphere object:

You should now have a pink ball and a white plane below it:

(The orange borders in the Scene window shows up when you select the Plane object in the Hierarchy window.)

Create a folder called Scripts in the Assets folder.

Create a C# script called Move inside the Scripts folder:

Drag the Move script on the Sphere object:

Select the Sphere object in the Hierarchy window and you should see the Move script in the Inspector window:

Click the Add Component button in the Inspector window to add a new component to the Sphere object.

Add a Rigidbody component:

Disable the Plane game object temporarily by ticking off the checkbox next to its name in the Inspector window and you will see the ball falling down:

Double-click the Move script in the Project window to open it.

Add the following code to it:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Move : MonoBehaviour
{
    // HERE
    private Rigidbody rb;

    void Start()
    {
        // HERE
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {

    }

    // START
    void FixedUpdate()
    {
        rb.AddForce(Vector3.right);
    }
    // END
}

The rb field holds a reference to the object Rigidbody:

private Rigidbody rb;

In the Start() function we get the Rigidbody component attached to the object using the GetComponent<type>() function:

rb = GetComponent<Rigidbody>();

In the FixedUpdate() function we apply a force to the ball:

rb.AddForce(Vector3.right);

This tells the engine that we want to "push" the ball along the world space red axis. Vector3.right is a shorthand for Vector3(1, 0, 0).

Click the Play button and you should see the ball rolling along the world space x-axis.

Let's move the ball using user input. Edit the Move script and add the following lines:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Move : MonoBehaviour
{
    private Rigidbody rb;

    // START
    private float horizontalInput;
    private float verticalInput;
    private float speed = 10.0f;
    // END

    void Start()
    {   
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {   
        // START
        horizontalInput = Input.GetAxis("Horizontal");
        verticalInput = Input.GetAxis("Vertical");
        // END
    }

    void FixedUpdate()
    {
        //rb.AddForce(Vector3.right);

        // HERE
        rb.AddForce(new Vector3(horizontalInput, 0.0f, verticalInput) * speed);
    }
}

First we create fields to store user input data and speed of the movement:

private float horizontalInput;
private float verticalInput;
private float speed = 10.0f;

In the Update() function we read the user input:

horizontalInput = Input.GetAxis("Horizontal");
verticalInput = Input.GetAxis("Vertical");

The Input.GetAxis function returns a value between -1 and 1. By default, "Horizontal" and "Vertical" axis are mapped to arrow keys.

In the FixedUpdate() function we use the input data and speed to push the ball:

rb.AddForce(new Vector3(horizontalInput, 0.0f, verticalInput) * speed);

Move the ball around using arrow (or WASD) keys.