Homepage showing a simple listing of title, featured image, then posted on

Sounds like there are two parts to your query: 1) Stop the first post from showing the full content, and 2) show the featured image on all the posts.

1) At the moment, the first post is using a different template to the other posts. To make the posts consistent, change this in index.php:

get_template_part( 'content' );

to this:

get_template_part( 'content', 'preview' );

2) Once all posts are using the same template, you’ll need to edit that template. Edit the file named content-preview.php. Directly below the code that starts with <?php the_title..., enter the following code to display your thumbnails:

<?php the_post_thumbnail(); ?>

Hope that helps 🙂