I need to make a Sphere appear 15 seconds after the FPSController has collided with a Cube and then make the Cube disappear after 30 seconds.

 public GameObject prefabSphere;
 
     void OnCollisionEnter(Collision other)
     {
         if(other.gameObject.CompareTag("Player"))
         {
             StartCoroutine("SpawnAndDie");
         }
     }
 
     IEnumerator SpawnAndDie()
     {
         yield return new WaitForSeconds(15f);
         Instantiate(prefabSphere);
         yield return new WaitForSeconds(15f);
         Destroy(gameObject);
     }

Really basic, but should get you started. http://docs.unity3d.com/ScriptReference/MonoBehaviour.StartCoroutine.html