adding custom stylesheet to wp-admin

According to WordPress Codex (here):

admin_enqueue_scripts is the first action hooked into the admin
scripts actions.

Example

Loading a CSS or JS files for all admin area:

//from functions.php

//First solution : one file
//If you're using a child theme you could use:
// get_stylesheet_directory_uri() instead of get_template_directory_uri()
add_action( 'admin_enqueue_scripts', 'load_admin_style' );
function load_admin_style() {
    wp_register_style( 'admin_css', get_template_directory_uri() . '/admin-style.css', false, '1.0.0' );
    //OR
    wp_enqueue_style( 'admin_css', get_template_directory_uri() . '/admin-style.css', false, '1.0.0' );
}

//Second solution : two or more files.
//If you're using a child theme you could use:
// get_stylesheet_directory_uri() instead of get_template_directory_uri()
add_action( 'admin_enqueue_scripts', 'load_admin_styles' );
function load_admin_styles() {
    wp_enqueue_style( 'admin_css_foo', get_template_directory_uri() . '/admin-style-foo.css', false, '1.0.0' );
    wp_enqueue_style( 'admin_css_bar', get_template_directory_uri() . '/admin-style-bar.css', false, '1.0.0' );
}

do i have to create folder in my plugins named css or do i just copy
my .css to the wp-admin/css directory?

No, put your CSS file together with the other, in your theme directory, then specify the path with:

get_template_directory_uri() . '/PATH_TO_YOUR_FILE'

For ex my file name is admin-style.css and i put it in a folder named css my path will looks like:

get_template_directory_uri() . '/css/admin-style.css'

Hope it helps!