Trim Post Content on Home Page

On your theme folder, open the index.php (or, home.php, or, front-page.php), and find, if there is a function named:

the_content()

Just change it to:

the_excerpt()

_{P.S.: If you can’t find the_content() in the mentioned file, try searching content.php, and in this case, change it with caution. Because in theme TwentyTwelve, it’s used with conditional tags for different pages.}