How to use Joystick from Standard Assets?

I am totally a beginner in Unity (Downloaded Unity 3 days ago). I created a simple game and I can move the player with keyboard by using this code -

 void FixedUpdate () {
             float speed = 250;
     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");
     
     Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
     GetComponent<Rigidbody>().AddForce (movement * speed * Time.deltaTime);
 }

However, I want to play the game in Android and for that I need to use joystick. Can anyone tell me how to use the joystick in Standard Assets?