I need help with a movement script

hi, I’m very new to coding and I’m trying to make a movement script for my first game and the script was working fine until I tried to add a jump. the jumping didn’t break the script but it does not work properly. could anyone please tell me how to make a jump script to this movement script:

[RequireComponent(typeof(Rigidbody))]
public class PlayerMovement : MonoBehaviour {

public Rigidbody rb;

public float jumpHeight = 10000f;

public float walkingSpeed = 5f;
public float runningSpeed = 10f;
float speed;

void Start ()
{
    rb = GetComponent<Rigidbody>();

    speed = walkingSpeed;

}

void Update ()
{
    if (Input.GetKey(KeyCode.LeftShift))
    {
        speed = runningSpeed;
    }
    else
    {
        speed = walkingSpeed;
    }

    float moveLR = Input.GetAxis("Horizontal") * speed;
    float moveFB = Input.GetAxis("Vertical") * speed;

    Vector3 movement = new Vector3(moveLR, 0, moveFB);

    rb.velocity = movement;

}

}

There is a player controller script in the free assets provided by unity, you should look at it.