Move Vehicle Forward

using UnityEngine;

public class PlayerController : MonoBehaviour
{

[SerializeField] float horsePower = 0f;
 private float turnSpeed = 35.0f;
    private float horizontalInput;
    private float forwardInput;

  private Rigidbody playerRb;

   private void Start()
    {
        playerRb = GetComponent<Rigidbody>();
    }
    void FixedUpdate()
    {
  //Move the vehicle forward
        playerRb.AddRelativeForce(Vector3.forward * forwardInput * horsePower);
        //have vehicle rotate as it turns
        transform.Rotate(Vector3.up * Time.deltaTime * turnSpeed * horizontalInput);
    }
}