New To WP – Basic Question About Post

I am new to using WP and am trying to put together my first blog.

I am using the default theme and have written my first couple of posts. What I would like to know is how do I set this default theme up so that on the main page it only shows a couple of paragraphs of the entire article?

I have the theme set for “summary” in the settings, but it doesn’t seem to be working the way I thought or would like it.

Any suggestions?