Out of the box the current default theme after installing WordPress shows all of your posts in the long format.
To change the theme to just show a short summary as per the screenshot below read on….
Go to the Dashboard -> Appearance -> Editor
Now edit the file called content.php by selecting it from selection on the right
Search for the term ‘the_content’ using ‘Ctrl + F’ on a Windows PC or ‘command + F’ on a Mac