Make menu horizontal

I’m trying to figure out how to make my main menu in a theme horizontal.

I’ve read various things and tried a plugin, all sayign that it makes the menu horizontal, but instead it is vertical every time.

Is it in the CSS or in the actual page? As you can tell I’m new to theme design.

I’m using this in the page:

<?php wp_list_categories('orderby=name&title_li=&depth=3'); ?>

Thanks