RollABall.cs

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {
    public float Speed;

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

    void FixedUpdate()
    {
        float moveHorizontal = Input.GetAxis("Horizontal");
        float moveVertical = Input.GetAxis("Vertical");

        Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
        rb.AddForce(movement*Speed);
    }
}