Different .php layout for each category

Hi,

Tried that already but seems it will only work if user is browsing on the category and not on post.

example:
http://mywebsite/category-2 <— works, but on
http://mywebsite/category-2/post1 <— doesnt work instead it is still using the template single.php.

On my site, I have static text and ads on all Post/Artciles of all the categories but I dont want to show this static text and ads on all the Post/Articles that are included in Category-2.

Is there a way to assign a specific layout (.php) for all the post within the category 12? Kindly give me simple steps to do because I am not well versed in .php.

Thanks a lot.