There are several ways to do what you want, here is one method that I used to use.
You have your landing page complete, so I assume you have a header with the navigation menus and other information on that page, you have some content in the middle of the page, and you have a footer at the bottom.
What you want to do is re-use the header and footer on all pages, and just change the content, so to do this, you could take all the code for your header and put that in a file and call it
header.php. Next, do the same with your footer, again naming it
footer.php. Finally, do the same thing with your content, calling it
Now, in your
landing_page.php is where you join all the files together:
output your content here
Now you can reuse your header and footer in other templates (about.php, etc) without copying all the header and footer code in each template.
This is a very basic example, and there are lots of other techniques but this should get you pointed in the right direction.
For a much more in-depth approach take a look at The No Framework Framework.