Making Character Rotate

Hello, I am making a game for a class and I am trying to make a character rotate to the left right or back if I push the left right or down button to make it easier to follow along with the game. Inserted below is my movement script I have so far.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovement : MonoBehaviour {
     public float speed = 6.0F;
     public float jumpSpeed = 8.0F;
     public float gravity = 20.0F;
     private float turningSpeed = 20f;
     private Rigidbody playerRigidbody;
     private Vector3 movement;
     private Vector3 moveDirection = Vector3.zero;
     void Update() {
         CharacterController controller = GetComponent<CharacterController>();
         if (controller.isGrounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             if (Input.GetButton("Jump"))
                 moveDirection.y = jumpSpeed;
             
         }
         
         moveDirection.y -= gravity * Time.deltaTime;
         controller.Move(moveDirection * Time.deltaTime);
     }
     
     void FixedUpdate () {
         //If the players movement vector does not = 0...
         if (movement != Vector3.zero) {
             // then create a target rotation based on the movement vector
             Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);
             
             //.... and create another rotation that moves from the current rotation to the taget rotation...
             Quaternion newRotation = Quaternion.Lerp (playerRigidbody.rotation, targetRotation, turningSpeed * Time.deltaTime);
             
             //...and change the players rotation to the new incremental rotation
             playerRigidbody.MoveRotation (newRotation); 
             
         }
     }
 }
 

`