Unity3D: Simple shooting function - Source: http://wiki.dreamsteep.com/Unity3d

Shoot.js

var prefabBullet: Transform;
var shootForce: float ;

function Update () {

   if(Input.GetButtonDown("Shoot")  ) 
   {
       var instancebullet =Instantiate( prefabBullet ,transform.position,Quaternion.identity);
	   instancebullet.rigidbody.AddForce(transform.forward * shootForce);
   }
   
}