NullReferenceException : Object reference not set to an instance of an object error

this is the error im getting trying to make a simple rolling ball game im very new to this mostly the codeing

my code so far im just trying to get simple movements the error is on line 18

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 * 10);


Most programming languages are case sensitive. start is not the same as Start. Your start method isn’t being called by Unity because it has the wrong name (it should be Start with an upper case S), so your rb field isn’t getting set to anything so it’s null.

And of course, whatever object this script is on must have a Rigidbody componnt.