The correct way to handle GET or POST requests in the admin panel

You should post some code examples, explaining what are you trying to do, to get more useful answers.

There are various hooks, and the one you should use depends on what you need. Anyway, here Codex: Actions Run During an Admin Page Request and here Codex: Administrative Actions the codex list the hooks to use for admin panel requests.

Example: If you have to activate something in the admin initialization, you can use the admin_init action hook, so the code will be: add_action('admin_init', 'your-custom-function'); If you have to operate when a page load in the dashboard, you can use the load-(page) hook; for example: add_action('load-post.php', 'your-custom-function'); and so on.

You can manage GET and POST requests with $_GET['x'] and $_POST['x'], where x is the variable you want to take from the query string. Example:

wp-admin/post.php?post=69
$_GET['post'] == 69