Q:

Completely customize admin menu without plugin

Basically what I want is to remove most of the admin menu and replace it with my own, without using a plugin. But I'd also like it to stay after a wp update.
Is this a possibility and if so, how?
Or is it just a million times easier to make the plugin and be done with it?
EDIT
In /wp-admin there is a menu on the left side. I won't be needing most of the menu and therefor don't want others to see or edit stuff in there(because the only thing that will happen is it will break the site). So I want to remove the unnecesary menu items and add relevant menu items. (It's about Posts, Pages, Media etc)

A:

As others already mentioned you could use the function remove_menu_page, but you still have to put this code somewhere.
If you really don't want to create a plugin you could add this code to the functions.php file of your theme.
In my opinion it's not theme related code, so it would be better to put it in a custom plugin. And it's really easy, here's a video on how to create one within minutes:
https://www.youtube.com/watch?v=S9Nhb1KX7vM
In your case it would look something like:
<?php
/*
Plugin Name:    Custom Admin Menu
Version:        1.0.0
Description:    My Custom Admin Menu
Author:         Peter van der Net
*/
if (!function_exists('my_custom_admin_menu')):

    function my_custom_admin_menu(){
        remove_menu_page('index.php');
        remove_menu_page('plugins.php');
        remove_menu_page('users.php');
        // etcetera..
    }

    add_action('admin_menu', 'my_custom_admin_menu');

endif;

/*?>*/

Put this code in a file named custom-admin-menu.php in the folder wp-content/plugins. And then activate the plugin.

