object reference not set to an instance error

hi I'm pretty new to Unity and I started to make a game, I was following many tutorial online, and tried to add a Boolean to start an animation for an ennemy but had a problem, this is the full script

using UnityEngine; using System.Collections; public class ennemi : MonoBehaviour { public Transform sightStart, sightEnd; public bool spotted = false; public GameObject arrow; public float speed = 1f;

 public Transform sightStart2, sightEnd2;
 public bool spotted2 = false;
 Animator anim;

 void start(){
     anim = GetComponent<Animator> ();
         }

 void Update () {

     Raycasting ();
     Raycasting2 ();
     Behaviours ();

     if (spotted2 == true) {
         anim.SetBool("attack", true);
     }
 }
 void Raycasting(){

     Debug.DrawLine(sightStart.position, sightEnd.position, Color.red);
     spotted = Physics.Linecast (sightStart.position, sightEnd.position, 1 << LayerMask.NameToLayer ("Player"));


 }
 void Raycasting2(){
     Debug.DrawLine(sightStart2.position, sightEnd2.position, Color.blue);
     spotted2 = Physics.Linecast (sightStart2.position, sightEnd2.position, 1 << LayerMask.NameToLayer ("Player"));
 
 
 }


 void Behaviours(){

     if (spotted == true) {
         arrow.SetActive(true);
         transform.Translate(Vector3.right * speed * Time.deltaTime);
     }
     else{
         arrow.SetActive(false);
     }
 }

     

}

and I get the error : Object reference not set to an instance of an object ennemi.Update () (at Assets/script/ennemi.cs:25)

and I dont know how to fix it, could you help me ?