Sort posts in descending order

Hello!
This is my first forum topic, I’ve done some googling but didn’t find a simple solution for this.
I need to have my posts in descending order, but not in all pages.
One of the pages I need them to be in descending order is this:
https://touhumaa.com/chevy49_2011/

The php in page template is this:

<?php $my_query = new WP_Query('category_name=proj_chevy49_2011&showposts=30');
	              while ($my_query->have_posts()) : $my_query->the_post();
	              $do_not_duplicate = $post->ID; ?>

I really don’t understand php at all, but I’m like a monkey, copying, pasting, trial and error learning…
I haven’t done the pages from the start, I had them made from my old html pages. Now I’m learning to use WordPress and customize the site.

So, do you guys there have any solutions for this…?