How to create a template in HTML?

I started creating web pages in plain HTML. I have a header, main content and a footer. The main content is divided into two, left and right content. Everything is the same on all pages except for the right content. Now I have about 15 pages. This is very tiresome if I make some changes on the other static pages (header, footer and left side content) as I have to go through all pages.

How can I create one HTML file for each static area and how am I going to integrate it?

There are, essentially, three places you can do this:

1. At build time. Run a script that puts the pages together and outputs static HTML. Then upload the static HTML to the server. Back in 2013 I recommended ttree, but these days static site builders are common and more powerful. My projects tend towards Gatsby (for complex projects) and Metalsmith (for simpler ones). Jekyll is very popular (and Github provides documentation for using it with GH Pages).
2. At runtime on the server. This could be a simple SSI, a PHP include or a full template system (of which there are many, including Template Toolkit and mustache), possibly running inside an MVC framework (such as Catalyst or Django). This is the most common approach, it has less time between making a change and putting the result life then doing the templating at build time.
3. At runtime on the client. This adds a dependency on client-side JavaScript (which can be fragile and unfriendly to search engines), so I wouldn't recommend it unless you already had a server-side or build-time solution in place. Gatsby, for example, is a static site generator that builds a React frontend backed by static pages.

There are a couple of ways to do this.

If you are working purely with HTML, then you can use a 'server side include' -- this is a tag which allows you to load a file into the page (see http://en.wikipedia.org/wiki/Server_Side_Includes). Whoever is hosting your website has to support this.

<!--#include virtual="../quote.txt" -->

If youe web host happen to use PHP, you can do this using the include method (see http://php.net/manual/en/function.include.php):

<?php include('path_to_file/file.htm');?>

This will include the file at the point you place the tag. But again, whoever is hosting your site needs to support this.

There are other methods of doing this, bit these are the two I make use of.