Basic "shoot to target" in Unity 3D

First, let's create a new Unity 3d Project, and select 3D.

Let's add a terrain GameObject-3D-Terrain.
Import Characters package Assets-Import Package-Characters, select all, and click Import.
Find the ThirdPersonController in the Project pane(use the search tool if you can't find it) and add it to the scene.

Create a Cube GameObject-3d Object-Cube. Name it ShootBox.
Create a Sphere GameObject-3d Object-Sphere, set its scale to 0.5 for x,y,and z. Name it BulletPlaceHolder.
Create a folder Resources.
Drag the two recently created objects into the Resources folder.
Create a Scripts folder.
Create three scripts: Bullet.cs, Player.cs,ShooterBox.cs, and add their respective code

Bullet.cs

using UnityEngine;
using System.Collections;

public class Bullet : MonoBehaviour
{
    private float Speed = 3.5f;
    public GameObject Target = null;
    // Use this for initialization
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        if (Target != null)
        {
            float step = Random.Range(1,5) * Time.deltaTime;
            this.transform.position = Vector3.MoveTowards(this.transform.position, Target.transform.position + new Vector3(0,0.5f,0), step);
        }
    }

    public void OnTriggerEnter(Collider other)
    {
        Debug.Log(other.name);
        Player playerComponent = other.GetComponent<Player>();
        if (playerComponent != null)
        {
            int newHealth = playerComponent.Health - 1;
            if (newHealth == 0)
            {
                Time.timeScale = 0;
            }
            else
                playerComponent.Health = newHealth;
            GameObject.Destroy(this.gameObject);
        }
    }
}

Player.cs

using UnityEngine;

using System.Collections;

using UnityEngine.UI;

public class Player : MonoBehaviour {

    public int Health = 100;

    public Text HealthText;

// Use this for initialization

void Start () {

}

// Update is called once per frame

void Update () {

}

    void OnGUI()

    {

        HealthText.text = Health.ToString();

    }

}

ShooterBox.cs

using UnityEngine;

using System.Collections;

public class ShooterBox : MonoBehaviour {

// Use this for initialization

    private GameObject player;

    private float lastTimeShooted = 0;

    public int ShootIntervalInSeconds = 15;

void Start () {

        this.player = GameObject.FindGameObjectWithTag("Player");

}

// Update is called once per frame

void Update () {

        if  ((lastTimeShooted + ShootIntervalInSeconds) < Time.time)

        {

            GameObject loadedResource = Resources.Load<GameObject>("BulletPlaceHolder");

            GameObject newInstance = GameObject.Instantiate<GameObject>(loadedResource);

            newInstance.transform.position = this.transform.position;

            Bullet bullet = newInstance.GetComponent<Bullet>();

            bullet.Target = this.player;

            lastTimeShooted = Time.time;

        }

}

}

Add the Bullet.cs script to the BulletPlaceHolder prefab.
Add the ShooterBox.cs script to the ShooterBox prefab.
Add the Player.cs script to the ThirdPersonController in the scene
Create a Canvas object and add two objects to it: HealthLabel, HealthText
Go to the ThirdPersonController in the scene, in the inspector find the Health Text field below the Player script and assign the recently created HealthText object.