Creating a Category Page

I'm new to Atahualpa and its single index.php template. I see references throughout the forum regarding category pages and archive pages, but I don't see how you create them in the first place. Very simply, how do I create a page that lists all recent posts in a particular category? Ideally, I'd like to have static text at the top, followed by a list of post titles or titles and descriptions. In other themes I've just popped some PHP code into the Page's text box, but that doesn't seem to work here.

Thanks in advance.