order and format of posts in category pages

When you do http://myblog.com/category/whatever it builds a dynamic page of all the posts that belong to category “whatever”. That’s fine, but…..

I would like a little more control on that. Mine essentially puts most of each post, and it does the oldest stuff first. Is there somewhere you can control what appears on those pages, and more importantly, the order which they appear. (I would like a smaller thumbnail image just the date of the post and I would like the newest ones to appear first) Is there some general template I need to edit for that?

Thanks, I’m sure that info is somewhere but it was not readily findable.