Автор, побольше конкретики.

using UnityEngine;
using System.Collections;

public class shoot : MonoBehaviour
{
public Transform _thePrefab;
public int forceSpeed;

// Use this for initialization
void Start()
{

}

// Update is called once per frame
void Update()
{

if(Input.GetMouseButtonDown(0))
{
Transform prefabInstance = (Transform)Instantiate(_thePrefab, GameObject.Find("Spawnpoint").transform.position,
Quaternion.identity);

prefabInstance.rigidbody.AddForce(transform.forward * forceSpeed);
}
}
}