Can't move character

I am making a game in Unity. I have written this script to make my character walk, but when i run the game i get an error: “Object Reference not set to an instance of an object”.

This is my code:
using UnityEngine;
using System.Collections;

public class Movement : MonoBehaviour
{

    public GameObject bullet;

    public float speed = 5;

    Rigidbody2D rb;


	// Use this for initialization
	void Start () {
        rb.GetComponent<Rigidbody2D>();
	
	}
	
	// Update is called once per frame
	void Update () {

        rb.velocity = Vector2.zero;

        if (Input.GetKey(KeyCode.A))
        {
            rb.velocity = Vector2.left * speed;
        }

        if (Input.GetKey(KeyCode.D))
        {
            rb.velocity = Vector2.left * speed;
        }

        if (Input.GetKey(KeyCode.Space))
            {
                GameObject go = Instantiate(bullet,
                    transform.position,
                    Quaternion.identity) as GameObject;
            }
        }
    }

The player-character does nothing when i press the keys “A” and “D”.

Your problem is in the Start method; change line 13 to ‘rb = GetComponent();’.