Link from navbar to categories

To show all categories on a page, you will have to create a new page template.

<?php
/**
 * Template Name: Categories Page
**/

get_header(); ?>

    <!-- content -->
    <div id="content">

        <ul class="category-list">
            <?php wp_list_categories( 'title_li=' ); ?>
        </ul>

    </div>
    <!-- / content -->

<?php get_footer();

Now create a new page for categories in WordPress admin and select this page template Categories Page from template list.

And add this page to your menu. That’s it.

Of course you will have to create CSS styles for appearances.