how to make a template for the post in each catagory in my custom theme

i made a theme for my own website and I’m wondering how you make the posts in a category have a different page layout and look than posts from another category. for example, I’m making a website selling themes and i want to make one category for the blog and another category, the themes. and i customized my single.php so it is like: the_excerpt is on the top in a big banner box, and my the_post is in the content section and that kind of layout.
but i want to have the posts under ‘blog’ to look different, like a simple header, with the title and the posts and comments and i don’t know how you do it. I’ve seen other websites do that but i don’t know how. any ideas? i was thinking having two files, one single.php and one viewthemes.php and single.php would be the blog template and viewthemes.php would be the template with the theme preview and the layout for that, but i dont know how to set it up so that the catagories would have different templates. please help me on this I’ve been stuck for about a month now trying to figure it out and everywhere i go, no one has answers,
thanks.