The variable bullet of 'Shooter' has not been assigned.

hi, i’m new to Unity and trying to learn some scripting.

i want to make a FPS shooter and watched some tutorials but i stil dont know how to fix his problem. when i want to shoot an error appears: The variable bullet of ‘Shooter’ has not been assigned. does someone know how to fix this?

using UnityEngine;
using System.Collections;

public class Shooter : MonoBehaviour {
	
	public GameObject bullet = null;
	public float bulletspeed = 500;
	
	private void Update () {
		if (Input.GetKeyDown(KeyCode.F))
		{
			GameObject shot = GameObject.Instantiate(bullet, transform.position + (transform.forward * 2), transform.rotation) as GameObject;
			
			shot.rigidbody.AddForce(transform.forward * bulletspeed);
		}
	}
	
	
}

Typically you’d drop a prefab onto the bullet game object slot in the Inspector window. The User Guide is really worth reading.