Getting variables from one class to another

Hello everyone, I've recently delved into Unity as a part of my class, we have been told to recreate a retro game throughout this month and the next.

HOWEVER!

I've been stuck for quite a while on how to edit a variable in another class then the one you're currently working in.

Normally I'd write "Class class;" and then have access to it just fine, however when ever i try and do this, it returns everything in my class as null.

How do i avoid this?

Heres the classes I've been toying around with:

 public Rigidbody bullet;
     public bool canMove = false;
     public float power = 1500;
     public float moveSpeed = 5.0f;
     
     // Use this for initialization
     void Start () 
     {
         //joint.connectedBody = GameObject.Find("physics_body").rigidbody;
         bullet = GameObject.Find("Cube").rigidbody;
     }
     
     // Update is called once per frame
     void Update () 
     {
         float h = Input.GetAxis("Horizontal") * Time.deltaTime * moveSpeed;
         float v = Input.GetAxis("Vertical") * Time.deltaTime * moveSpeed;
         
         transform.Translate(h, v, 0);
         
         
         if(canMove == true)
         {
             if(Input.GetButtonDown("Fire1"))
             {
                 
                 Shoot();
             }
         }
     }
     
     void Shoot()
     {
           
         Rigidbody instance = Instantiate(bullet, transform.position, transform.rotation) as Rigidbody;
             Vector3 fwd = transform.TransformDirection(Vector3.forward);
             instance.AddForce(fwd * power);
     }

This code basically just allows a player to choose in front of him/her, however since I've been trying to make a menu, this needs to be "paused" when ever the player is on the menu.

So i figured i could just do this:

 bool canMove = false;
     ShootBullet shoot;
     // Use this for initialization
     void Start () 
     {
     
     }
 
     // Update is called once per frame
     void Update () 
     {
         if(canMove == true)
         {
             shoot.canMove = true;
         }
         if(Input.GetKeyDown(KeyCode.B))
         {
             canMove = true;
         }
     }

But no, it just returns everything as null.

Hopefully someone can help me and point out where I've gone wrong.

Thanks.