I am new to unity and am trying to do the roll-a-ball game, but when I try to run it, I get this error: NullReferenceException: Object reference not set to an instance of an object. What am I doing wrong?

 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);
     }
 }