How do I fix this error?

I’m a complete beginner and I’m just following a tutorial on Youtube.

It keeps on saying that there's an error with the script ![87222-assembly-csharp-scripts-playermovementcs-monodevel.png|1680x987](upload://lJirTaffitXLxI0OD4GxAl2A0QO.png)

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {
	public float moveSpeed;

	private Vector3 input;

	// Use this for initialization
	void Start () {

	}

	void Update () {
		input = new Vector3(Input.GetAxis ("Horizontal"), 0, Input.GetAxis ("Vertical"));
		rigidbody.AddForce(input);
	}
}

The variable “rigidbody” is missing, also the part where it’s initialized You probably forgot to type these lines.