I'm trying to allow my character to only jump when grounded

I’m trying to only allow my character to jump only while grounded and I’m not exactly sure how to do that any ideas on how I can implement it I don’t really know coding that well.

//my code

using UnityEngine;

public class PlayerController : MonoBehaviour {

private float speed = 7f;
private float lookSensitivity = 3f;
private float jumpForce = 10.0f;

private PlayerMotor motor;
private Rigidbody rb;

private void Start()
    motor = GetComponent<PlayerMotor>();
    rb = GetComponent<Rigidbody>();

private void Update()
    float xMov = Input.GetAxisRaw("Horizontal");
    float zMov = Input.GetAxisRaw("Vertical");

    Vector3 movHorizontal = transform.right * xMov;
    Vector3 movVertical = transform.forward * zMov;   

    Vector3 movement = new Vector3(xMov, 0, zMov);

    rb.AddForce(movement / speed);

    if (Input.GetKeyDown(KeyCode.Space))
        rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);

    Vector3 velocity = (movHorizontal + movVertical).normalized * speed;


    float yRot = Input.GetAxisRaw("Mouse X");

    Vector3 rotation = new Vector3(0f, yRot, 0f) * lookSensitivity;


    float xRot = Input.GetAxisRaw("Mouse Y");

    Vector3 cameraRotation = new Vector3(xRot, 0f, 0f) * lookSensitivity;



Declare your variable

bool isGrounded = true;

Modify your if statement

if (Input.GetKeyDown(KeyCode.Space) && isGrounded){
         rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
         isGrounded = false;

Add collision check function

void OnCollisionEnter (Collision collisionInfo){
      isGrounded = true;