Displaying Categories

I’m trying to display categories in my sidebar but am having trouble.

I know there’s a widget that does it, but I like to edit the style of text etc and the widget won’t let me do that.

So I can display categories by using

<?php the_category(‘ ‘); ?>

but that only displays the categories of the most recent post.

So I tried using

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php the_category(‘ ‘); ?>
<?php endwhile; else: ?>
<?php endif; ?>

but that displays categories multiple times.

Is there something like <?php the_category(‘ ‘); ?> I can use to display all categories at all times?

Thanks!