how to shoot without pressing a button automatically continue to fire on startup

using UnityEngine;
using System.Collections;

public class Done_Boundary
public float xMin, xMax, zMin, zMax;

public class Done_PlayerController : MonoBehaviour
public float speed;
public float tilt;
public Done_Boundary boundary;

public GameObject shot;
public Transform shotSpawn;
public float fireRate;
private float nextFire;

void Start ()
		nextFire = Time.time + fireRate;
		Instantiate(shot, shotSpawn.position, shotSpawn.rotation);
		GetComponent<AudioSource>().Play ();

void FixedUpdate ()
	float moveHorizontal = Input.GetAxis ("Horizontal");
	float moveVertical = Input.GetAxis ("Vertical");

	Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
	GetComponent<Rigidbody>().velocity = movement * speed;
	GetComponent<Rigidbody>().position = new Vector3
		Mathf.Clamp (GetComponent<Rigidbody>().position.x, boundary.xMin, boundary.xMax), 
		Mathf.Clamp (GetComponent<Rigidbody>().position.z, boundary.zMin, boundary.zMax)
	GetComponent<Rigidbody>().rotation = Quaternion.Euler (0.0f, 0.0f, GetComponent<Rigidbody>().velocity.x * -tilt);


put it in Update Method;