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If these files are not available in your Theme's directory WordPress will look for archive.php and single.php, respectively. If even these files are not present it will default to index.php. See Template Hierarchy for more details.
Make a copy of the single.php or archive.php file as a starting point. That way they will inherit some of the theme's html structure. Remember to add the post type name to the file ex. archive-post_type.php. From there you may want to customize further...
If you are using custom taxonomies with your post type and would like them to display in your template file use the_terms().
If a given custom post type is hierarchical in nature, then wp_list_pages() can be used to list the member of that custom post type. wp_list_pages().
Fun with conditional statements... Conditional tags for Custom Post Types
Template Hierarchy: Category Templates, Tag Templates, Taxonomy Templates, Page Templates, Post Type Templates, Author Templates, Date Templates, Search Templates, 404 Templates, Attachment Templates, Loop Templates