Looking and firing towards mouse position, 2d game.

I am making a 2d game where I need my player to fire towards my mouse position when the space key is pressed. I already have the shooting that works, but the aiming towards the mouse is where I get stuck.

Thank you very much for your help!

 using UnityEngine;
 using System.Collections;
 
 public class PlayerShooting : MonoBehaviour 
 {    public GameObject bulletPrefab;
     public Transform bulletSpawn;
     public float coolDown =  3f;
 
     void Update () 
     {
         //print ("the cooldown time is " + coolDown);
         CoolDownTimer();
         if(coolDown <= 0)
         {        
             if(Input.GetKeyDown(KeyCode.Space))
             {
                 Instantiate(bulletPrefab,bulletSpawn.position,bulletSpawn.rotation);
                 coolDown = 1f;
             }
         }
     }
 
     void CoolDownTimer()
     {
         coolDown -= Time.deltaTime;
     }
 }