Have a look at the first-person shooter tutorial.

public var snowballPrefab : Transform;
public var snowballSpeed : float = 6000;

function Update(){
    if(Input.GetButton("Fire1"))
        {
            if(!snowballPrefab || !snowballSpeed)
            {
                Debug.Log("[Shoot] 'bulletPrefab' or 'bulletSpeed' is undefined");
            }else{
                var snowballCreate = Instantiate(snowballPrefab, GameObject.Find("SSpawnPoint").transform.position, Quaternion.identity);
                snowballCreate.rigidbody.AddForce(transform.forward * snowballSpeed);
            }
        }
}

Try this, replace snowball with your game object. I am still trying to find out a better way:)

put A empty game object in front of your gun ( where u want to shoot from) and name it "spawnPoint" then attach this script:

var fireRate : float = 0.1;
var Prefab: Transform;
private var nextFire = 0.0;
var speed : float = 50;
function Update () {
    if(Input.GetKey("mouse 0")&&Time.time > nextFire){
    nextFire = Time.time + fireRate;
    var copy = Instantiate(Prefab,GameObject.Find("spawnPoint").transform.position,transform.rotation);
    copy.rigidbody.velocity = transform.TransformDirection(Vector3.forward * speed);

}

}

to the player, you can edit the fire rate, in the inspecter panel make sure to when it says prefab, add a game object there, such as a cube or sphere.

dont ask for scripts use the references and learn it yourself