Go to ‘appearance’ and then ‘editor’. In the next screen select the file called ‘index.php’. In the code you will run into the_content (use control+F). Replace it with the_excerpt.
Make sure to copy the whole file to notepad or so in case something goes wrong, so you can paste the original code back. Alternally, you can of course replace the file using FTP when needed, but this operation is but a small one.
When you have the_excerpt all posts will show 30 words (I believe) unless you use the “exerpt” foldout menu in the write/edit screen.