Script to toggle a flashlight on and off. Attach this component to an object with a spotlight component. Add an AudioSource so the switch sound will play. Also remember to adjust the ambient color of the scene for darkness: (Edit->Render Settings->Ambient color).

Flashlight.cs

using UnityEngine;
using System.Collections;

public class Flashlight : MonoBehaviour {
	public AudioClip switchSound;
	
	void Update () {
		if(Input.GetKeyDown(KeyCode.F))
		{
			audio.PlayOneShot(switchSound);
			
			if(light.enabled)
			{
				light.enabled = false;
			}
			else
			{
				light.enabled = true;
			}
		}
	}
}