Create an object on mouse click?

Zero understanding of how to script, please talk to me like I'm dumb.
I (after much tutorialing) am able to pick up an object and give it a specific tag while it is held ("inhand") - I want to be able to spawn another object into the world by clicking while the first object is held/"inhand."
The test-script I made (based on trying to divine JavaScript logic from the Unity manual) is trying to spawn a cube when the mouse is clicked and the other object is tagged "inhand" - it (obviously) doesn't work because it (clearly) makes no sense. Please tell me why it makes no sense, and then please tell me how to script the above.
#pragma strict

 function Start () {
 
 }
 
 function Update () {
 
 }
 
 function onClick () {
     if (gameObject.CompareTag ("inhand")) {
         var cube : GameObject  = GameObject.CreatePrimitive(PrimitiveType.Cube);
         cube.transform.position = Vector3(0, 0.5, 0);
         }
 }

(Note: Ideally, I want to spawn a different object that is NOT a cube, so what would I change in the proper code? Also, I want it to spawn in a location relative to the FPSController, not in a set location in the world. ALSO, I don't know what object to attach the script to once it's finished (again, no knowledge). If you can help, thanks!)