how to change control to fire from keyboard to Button UI

hello, i have a problem. i have a code like this

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class Fire : MonoBehaviour {
public GameObject Bullet_Emitter;
public GameObject Bullet;
public float Bullet_Forward_Force;

// Use this for initialization
void Start () {

// Update is called once per frame
void Update ()
	**if (Input.GetButtonDown("Fire1"))**
		//The Bullet instantiation happens here.
		GameObject Temporary_Bullet_Handler;
		Temporary_Bullet_Handler = Instantiate(Bullet,Bullet_Emitter.transform.position,Bullet_Emitter.transform.rotation) as GameObject;

		//Sometimes bullets may appear rotated incorrectly due to the way its pivot was set from the original modeling package.
		//This is EASILY corrected here, you might have to rotate it from a different axis and or angle based on your particular mesh.
		Temporary_Bullet_Handler.transform.Rotate(Vector3.left * -60);

		//Retrieve the Rigidbody component from the instantiated Bullet and control it.
		Rigidbody Temporary_RigidBody;
		Temporary_RigidBody = Temporary_Bullet_Handler.GetComponent<Rigidbody>();

		//Tell the bullet to be "pushed" forward by an amount set by Bullet_Forward_Force.
		Temporary_RigidBody.AddForce(transform.forward * Bullet_Forward_Force);

		//Basic Clean Up, set the Bullets to self destruct after 10 Seconds, I am being VERY generous here, normally 3 seconds is plenty.
		Destroy(Temporary_Bullet_Handler, 10.0f);


in this code my karakter fire with keyboard left ctrl or klik mouse. i want change to Button UI. because this is for android. plese help me and sorry for bad english. thx

Try something like this.

//add this at the top of your script
using UnityEngine.UI;

//put this in your class...

public Button fireButton; //<--drag/drop your UI button onto this in the inspector

void Start()
     //add an onclick event to your UI button
     fireButton.onClick.AddListener(() => Shoot());

void Shoot()
     //do your shooting stuff

Thanks a lot bro you’re great…

For me its not working.
NullReferenceException: Object reference not set to an instance of an object
bulletfire.Start () (at Assets/bulletfire.cs:25)

e.g, cs:25 firButton.onClick.AddListener(() => Shoot());

plz help,thanks.