instantiate object on tap

Instantiation

http://unity3d.com/support/documentation/ScriptReference/Object.Instantiate.html

Touch

http://unity3d.com/support/documentation/ScriptReference/TouchPhase.html

Basic touch and Instantiation, not optimized and not suggested for practical use in it's basic state:

 using UnityEngine;
 using System.Collections;

 public class touchSpawnExample : MonoBehaviour {
     
     public GameObject itemToSpawn;
     private GameObject itemToSpawnInstance;
     
     void Update()
     {
         if( Input.touchCount == 1 )
         {
             Touch touch = Input.touches[0];
             
             switch( touch.phase )
             {
             case TouchPhase.Began:
                 itemToSpawnInstance = (GameObject)Instantiate( itemToSpawn, Vector3.zero, Quaternion.identity );
                 break;
             case TouchPhase.Moved:
                 // track touch movement and doStuff here
                 break;
             case TouchPhase.Ended:
                 // track touch lifted off screen and doStuff here
                 break;
             default:
                 break;
             }
         }
     }
 }