where/how can I edit the layout on a category page?

[eluser]girlLAKE62[/eluser]
Hi,
Excuse me if I ask a stupid question. I'm working on my first codeigniter project and so far I think it's great.
I've been brought on board this project to change the format of the blog/category feed on a live site.
This live site has several categories, including "blog" which is set up as a category and that is how they want it. Some of the category pages are styled to display the title, image and excerpt of the first three entries for that category. I would like to style the "blog" category to display the same way.
1. How and where can I change the layout of that one category page? I imagine I can borrow some of the code for the already styled categories for the blog category.
2. How and where can I edit that feed so that the first 6 entries are displayed?
Thanks in advance for your help!