Error! please help 'object reference not set...'

Hello everyone,

I have just got unity and am pretty new, I have been following this tutorial… Unity 5 - Roll a Ball game - 2 of 8: Moving the Player - Unity Official Tutorials - YouTube
it is great and have been learning lots of new stuff, but I am getting this error in my code, please help… “NullReferenceException: Object reference not set to an instance of an object
PlayerController.FixedUpdate () (at Assets/Scripts/PlayerController.cs:22)”

I dont know what I am doing wrong, I have triple checked my code and it seems perfect, here is my code…

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);


Thanks for any help!

void start ()

Start() must have a capital “S”, otherwise this method is not called automatically and the variable “rb” remains uninitialized.