editing categories

Hi,

So I would like to change the layout of categories on my twenty twelve theme. Essentially, I want the latest post to be large at the top and then the rest would be filtered into a grid of images with text. Something similar to this: http://www.ruemag.com/category/fashion-style/snag-her-style

My site is http://www.totemag.com

I’ve been reading up on this but I’m very confused. I think I have to mess with the category.php but I don’t really understand what exactly I would change and if I change it to something how the .php file will understand what to do. I’ve spent days researching this before I came to the forum.

So if anyone could help that would be great!