first time coding, getting "NullReferenceException: Object reference not set to an instance of an object" error

making the beginner rollaball game from the tutorials and I copied the code exactly as they show it and I keep getting this error
NullReferenceException: Object reference not set to an instance of an object

heres the 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);

just trying to get the ball to move, its saying the “rb.AddForce (movement);” is where im having an error

any help would be greatly appreciated!

The “standard” Start function, recognized by Unity, has the capital S.

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

Actually, your code skips that, and rb is not initialized.