Creating your template
You should only use a template, if you really need it. This means if you cannot modify the output using only CSS. This would be the case if you want to hide e.g. the secondary menu, which is still not a block by default but always rendered through page.html.twig. (formerly page.tpl.php)
If you know the names of the templates you need from Drupal 7, it will be easy for you. Most of the time, the name of your template will have the same name. Only the .tpl.php file ending is replaced by .html.twig.