How to create a custom wordpress plugin

I’m learning plugin development and need to create a plugin that contains:

1. Add menu item and page in Admin Panel of WordPress
2. on that page, let us add text messages using a form having a text area and button.
3. it should show the added text messages as message boxes in the WordPress admin panel.
4. it should let us add/delete/update the messages.

---

<?php
/*
Plugin Name: The Admin Messages
Plugin URI: https://wpsaints.com
Author: Wp saints
Author URI: https://wpsaints.com
Version: 9.0
*/

function the_admin_menu(){
    add_menu_page('Form' , 'Form Items' , 'manage_options'  ,'mypage','the_admin_menu_page'  , ' dashicons-menu' , 6);

}
add_action('admin_menu' ,'the_admin_menu');


function the_admin_menu_page()
{

?>
<form method="post">
<br><br>
<lable><b>Enter your Message</b></lable><br><br>
<textarea></textarea><br><br>
<button> Submit</button>


</form>

<?php

function jal_install () {
   global $wpdb;

   $table_name = $wpdb->giftmix . "messagebox"; 
}
global $wpdb;

$charset_collate = $wpdb->get_charset_collate();

$sql = "CREATE TABLE $entermessage (
  id mediumint(9) NOT NULL AUTO_INCREMENT,
  time datetime DEFAULT '0000-00-00 00:00:00' NOT NULL,
  name tinytext NOT NULL,
  text text NOT NULL,
  url varchar(55) DEFAULT '' NOT NULL,
  PRIMARY KEY  (id)
) $charset_collate;";

require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
dbDelta( $sql );
  if($_SERVER['REQUEST_METHOD'] == "POST")
  {


 $Enter = ($_POST["YourName"]);
      $Email = ($_POST["Email"]);
}


}


?>

I have created this but now need suggestions from seniours on what to do next?
Please Help. Thank you.