Puting a link on the catagory & search page to the post

Do not edit the Twenty Twelve theme. It is the current default WordPress theme and having access to an original, unedited, copy of the theme is vital in many situations. First create a child theme for your changes. Or install a custom CSS plugin.

I’ve already got a child theme. I just need to know where and what code to put in, a recommendation on a plugin.

I assume the category.php and search.php but I have no clue about what code.