Wordpress editable template fields

I created several template files like this:

<?php
/*
Template Name: My Template 1
*/

get_header('my-template-1');
while ( have_posts() ) : the_post();
  the_content();
endwhile;
get_footer('my-template-1');

?>

I can assign a template to each page of my site.

Question:

How can I make specific portions of these templates to be editable in admin?

For example, I wish to have a field (which is assigned to one template) with some specific text in it, and if I edit and save this field - the updated text should appear in all pages that use this specific template.