Find Out Which Template To Edit/Create
tpl.php files are called templates in the Drupal system. They render the HTML that you see on any given page, may it be a block/view/panel or anything. 

Theme Developer is a popular module used to pinpoint the template file that's in play. I suggest you try it to better understand how template system works.
To understand more on customizing templates and naming conventions read through this guide on working with template suggestions.

If its a view you want to modify- Use views GUI to the maximum extent(changing the sort order, re-arranging fields etc.). Still not getting what you want ? Override view templates for maximum control. 

(Helpful Articles on overriding view templates)

http://www.wdtutorials.com/drupal/views/how-to-create-custom-template-view-drupal-tutorial-article

https://www.interworks.com/blog/carmstrong/2013/02/27/quick-tutorial-overriding-drupal-views-using-templates