Look somewhere after the “Loop” starts (it starts with <?php while( have_posts() ): the_post(); ?>) in your index.php file.
Find <?php the_content(); ?> and change it to <?php the_excerpt(); ?>. If you don’t want any content, just remove the_content() all together.
If you’re using a theme from wordpress.org, it would best to do this in a child theme, so you can keep the original theme up to date.