How do I add Javascript and CSS files into WordPress?

For your purpose enqueue is the key:

• wp_enqueue_script()
• wp_enqueue_style()

You can consult this question: How to enqueue the style using wp_enqueue_style() for style enqueue.

For scripts enqueue I recently did as:

// ENQUEUE JAVASCRIPTS FOR CUSTOM PURPOSE
function scripts_for_something() {

    wp_register_script( 'my-scripts', get_template_directory_uri() . '/js/my-scripts.js' );

    wp_enqueue_script( 'my-scripts' );
}

add_action( 'admin_enqueue_scripts', 'scripts_for_something' );

admin_enqueue_scripts will enqueue the scripts in the admin panels only. You can use wp_enqueue_scripts instead for front-end.