How to add event to an object

Hi, I'm trying to create an 2D game in unity and I have some problems. I want to create the menu of the game, and I created the buttons as images to add scripts to each button to do something. I've created this script to the button "Start Game":

 using System.Collections;
 
 public class Jogo : MonoBehaviour
 {
 public string scene;
 private bool loadLock=false;
 
 void Update ()
 {
 
 if (Input.GetMouseButtonDown (0) && !loadLock)
 
 {
 LoadScene();
 
 }
 
 }
 
 void LoadScene()
 
 {
 if (scene == null)
 
 return;
 
 loadLock = true;
 
 Application.LoadLevel (scene);
 }
 }

And I add this script to the button, but this script is not only executed when I clicked the button but is executed anywhere I click. How I resolve this?