Articles will be listed on a category page according to their Publish Date in descending order. This means that your most recently published article will appear at the top of the page.
How to change the article order on a category page
You can change the order article teasers are listed in on a category page by editing the the Publish Date of individual articles.
To have an article appear first on a category page, edit its Publish Date to the most recent date.
If you're adding a collection of articles and want to manage the order of their appearance, then add the article meant to be last in the list - first.