Finding a game object?

I am trying to make a homing bullet that follows the enemies in my game. At the start, I want it to randomly choose an enemy that is active in the scene (the enemies are all clones of the same object), and then follow it.

private var enemy : GameObject;

function Start () {
    enemy = GameObject.Find("Enemy_(Clone)");

function Update() {
    transform.LookAt(enemy);transform.Translate(Vector3.forward * 10 * Time.deltaTime); Destroy (gameObject, 5);

But, I get the error "No appropriate version of 'UnityEngine.Transform.LookAt' for the argument list '(UnityEngine.GameObject)' was found."

what is wrong?

The error message is because you are passing the wrong type to LookAt. It is expecting a Vector3 or a Transform. e.g.

transform.LookAt( enemy.transform );