GameObject Drag and Drop action for Unity3D

DragAndDrop.cs

using UnityEngine;
using System.Collections;

public class DragAndDrop : MonoBehaviour
{  
	private bool _mouseState;
	public GameObject Target;
	public Vector3 screenSpace;
	public Vector3 offset;

	// Use this for initialization
	void Start ()
	{
	
	}
	
	// Update is called once per frame
	void Update ()
	{
		// Debug.Log(_mouseState);
		if (Input.GetMouseButtonDown (0)) {
			RaycastHit hitInfo;
			if (Target == GetClickedObject (out hitInfo)) {
				_mouseState = true;
				screenSpace = Camera.main.WorldToScreenPoint (Target.transform.position);
				offset = Target.transform.position - Camera.main.ScreenToWorldPoint (new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenSpace.z));
			}
		}
		if (Input.GetMouseButtonUp (0)) {
			_mouseState = false;
		}
		if (_mouseState) {
			//keep track of the mouse position
			var curScreenSpace = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenSpace.z);

			//convert the screen mouse position to world point and adjust with offset
			var curPosition = Camera.main.ScreenToWorldPoint (curScreenSpace) + offset;

			//update the position of the object in the world
			Target.transform.position = curPosition;
		}
	}
	
	
	GameObject GetClickedObject (out RaycastHit hit)
	{
		GameObject target = null;
		Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
		if (Physics.Raycast (ray.origin, ray.direction * 10, out hit)) {
			target = hit.collider.gameObject;
		}

		return target;
	}
}

Something which worked. Thank you.
I didn't know where to add this script so I added it to my floor plane of the scene.