List posts per category

In the following code I managed to display all the custom categories and posts under the custom page I created(page-portfolio-template.php).

<?php
    $args = array('post_type' => 'portfolio', 'post_per_page' => 3);
    $loop = new WP_Query( $args );
    if ( $loop->have_posts() ) :
        while ( $loop->have_posts() ) : $loop->the_post(); ?>
            <div class="thumbnail"><?php the_post_thumbnail('medium'); ?> </div>
            <h1><a href="<?php the_permalink(); ?>"> <?php the_title(); ?></a></h1>
            <p><?php the_category();?></p>
            <p> <?php the_content(); ?> </p>
            <small>Posted on: <?php the_time('F j,Y'); ?> at <?php the_time('g:i a'); ?></small>
        <?php endwhile; // end while
    endif;
    ?>

What I”m struggling to think of, is:How can I print only the posts of the categories that I click?
Eg:If I click on reviews,then it prints all the posts under reviews only.

At the moment if I click on a category it only display the category title.