You have noticed that if you create a new page some fields are predefined; for example, the new page is active and shown in the menu. Page-specific metadata is prefilled with a comment:
<!-- Add code here that should appear in the metadata section of all new pages -->
You can change the predefined values in the admin console. In the admin console, click on Site Admin | Page Defaults and set the fields the way you would like them to be set if a new page is created.
You can predefine meta tags and even the content field of the page. For example, you can create empty meta tags, which are shown as follows:
<meta name="keywords" content="" /> <meta name="description" content="" />
Now, the editor of the content has to just fill the content attribute of the meta tags (between the quotes) while creating or editing pages.