Nullreferenceexception on roll a ball project

I am incredibly new to unity, just downloaded it this morning and jumped right in with the roll a ball project. I'm starting to get the impression this was the wrong place to start. I have followed the tutorial step by step and copied his code word for word but am still getting this error message:

NullReferenceException: Object reference not set to an instance of an object PlayerController.FixedUpdate () (at Assets/scripts/PlayerController.cs:22)

Here is the script:

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);
 }

}

Now, im gonna take myself off and learn a bit about scripting before even coming back to this thing, but for the sake of my sanity, can someone please tell me where i went wrong?