Drag-and-drop problem

Hi. I have a problem about Drag-and-drop object. Below is my script attached to my object. I want player must click on Object to move it but now they can click anywhere on screen and then move the object. And the second problem is, if I have 2 objects, if I move an object, the rest also move with first object.

 using UnityEngine;
 using System.Collections;
 
 public class DragDrop2 : MonoBehaviour {
 
     private Vector3 screenPoint;
     private Vector3 offset;
 
     void Update(){
 
                 if (Input.GetButtonDown ("Fire1")) {
             
             screenPoint = Camera.main.WorldToScreenPoint (transform.position);
             
             offset = transform.position - Camera.main.ScreenToWorldPoint (new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
             
                         Screen.showCursor = false;
                 }
 
                 if (Input.GetButton ("Fire1")) {
             
                         Vector3 curScreenPoint = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
             
                         Vector3 curPosition = Camera.main.ScreenToWorldPoint (curScreenPoint) + offset;
             
                         transform.position = curPosition;
                 }
                         
                         if (Input.GetButtonUp ("Fire1")) {
             
             Screen.showCursor=true;
             
                         }
         }
 }