How To Include CSS and jQuery in my WordPress plugin?

Very Simple:

Adding JS/CSS in the Front End:

function enqueue_related_pages_scripts_and_styles(){
        wp_enqueue_style('related-styles', plugins_url('/css/bootstrap.min.css', __FILE__));
        wp_enqueue_script('releated-script', plugins_url( '/js/custom.js' , __FILE__ ), array('jquery','jquery-ui-droppable','jquery-ui-draggable', 'jquery-ui-sortable'));
    }
add_action('wp_enqueue_scripts','enqueue_related_pages_scripts_and_styles');

Adding JS/CSS in WP Admin Area:

function enqueue_related_pages_scripts_and_styles(){
        wp_enqueue_style('related-pages-admin-styles',  get_stylesheet_directory_uri() . '/admin-related-pages-styles.css');
        wp_enqueue_script('releated-pages-admin-script', plugins_url( '/js/custom.js' , __FILE__ ), array('jquery','jquery-ui-droppable','jquery-ui-draggable', 'jquery-ui-sortable'));
    }
add_action('admin_enqueue_scripts','enqueue_related_pages_scripts_and_styles');

You can use the following function to enqueue script or style from plugin.

function my_enqueued_assets() {
    wp_enqueue_script('my-js-file', plugin_dir_url(__FILE__) . '/js/script.js', '', time());
    wp_enqueue_style('my-css-file', plugin_dir_url(__FILE__) . '/css/style.css', '', time());
}
add_action('wp_enqueue_scripts', 'my_enqueued_assets');

You can add scripts and css in back end and front end with this following code: This is simple class and the functions are called in object oriented way.

class AtiBlogTest {
function register(){
    //for backend
    add_action( 'admin_enqueue_scripts', array($this,'backendEnqueue'));
    //for frontend
    add_action( 'wp_enqueue_scripts', array($this,'frontendEnqueue'));
}
function backendEnqueue(){
    wp_enqueue_style( 'AtiBlogTestStyle', plugins_url( '/assets/css/admin_mystyle.css', __FILE__ ));
    wp_enqueue_script( 'AtiBlogTestScript', plugins_url( '/assets/js/admin_myscript.js', __FILE__ ));
}
function frontendEnqueue(){
    wp_enqueue_style( 'AtiBlogTestStyle', plugins_url( '/assets/css/front_mystyle.css', __FILE__ ));
    wp_enqueue_script( 'AtiBlogTestScript', plugins_url( '/assets/js/front_myscript.js', __FILE__ ));
}
}

if(class_exists('AtiBlogTest')){
$atiblogtest=new AtiBlogTest();
$atiblogtest->register();
}