Would like to add category.php and tag.php

Hi, I would like to add further formatting. Specifically, I'd like to create a category.php and a tag.php. I would like then to use these templates to format the pages differently. How would I add these into the theme? To which file would I add the call for the specific template? I do not see a specific template that governs either category or tags, so I don't know how to call these templates.