Create full-width page template

YES! I am using a child theme.
I have been adding customizations under:

EDIT THEMES tab
Click on “Stylesheet (style.css)”
Putting blocks of customization code into this window, then clicking “Update File.”

Sorry… don’t understand. I’m totally NEW to all of this.
I thought I was supposed to create a new .php file in order to make a Fullwidth page template. So I opened a new .php file (which I’ve called “Fullwidth Page”), put in this code at the top:

<?php
/*
Template Name: Fullwidth page
*/
?>

And now I have NO IDEA what else I’m supposed to put in this .php file.

I *thought* what I am trying to do is create a template, which will then show up as an option when I create a page, and I’ll have the option to choose “Fullwidth page” which will be pre-formatted to the content width I want.

But I don’t know where to find the code to describe what I want on my Fullwidth page.
I copied the code from the TwentyEleven page.php, but it doesn’t seem to mention anything about margins:

get_header(); ?>

		<div id="primary">
			<div id="content" role="main">

				<?php while ( have_posts() ) : the_post(); ?>

					<?php get_template_part( 'content', 'page' ); ?>

					<?php comments_template( '', true ); ?>

				<?php endwhile; // end of the loop. ?>

			</div><!-- #content -->
		</div><!-- #primary -->

<?php get_footer(); ?>

I need to get walked through this REAL slow. Things which may seem obvious to others are still unknown to me, have only been doing this for a few days.
Thanks so much!!