How do i fix this nullreferenceException

Hi, i am very new to unity and i just tried to make a block dodge game posted to youtube (Brackeys How to make a Dodge The Blocks game (Livestream) - Unity Tutorial - YouTube) evrything went good until i begun coding, i believed i filled out the exact same as he did, but i got an error saying: NullReferenceException: object not set to an intance of an object Player.fixedupdate () (atassets/player.cs:19) line 19 is the

 rb.MovePosition(rb.position + Vector2.right * x);

using UnityEngine;
using System.Collections;

public class Player : MonoBehaviour {

public float speed = 15f;

private Rigidbody2D rb;

void start ()
{
    rb = GetComponent<Rigidbody2D>();
}

void FixedUpdate ()
{
    float x = Input.GetAxis("Horizontal") * Time.fixedDeltaTime * speed;

    rb.MovePosition(rb.position + Vector2.right * x);

}   

}

This error is probably due to rb == null.
Please be sure that the game object with your script has Rigidbody2D component.

Also - write “Start” with capital “S”. It’s mistake to write it with lowercase letter.