To active the post summary, go to Setting - Reading and set the "For each article in a feed, show" to Summary.
Our newer themes also allow you to control the excerpt length with the "Excerpt length" option in Appearance - Theme Options - Page Setup.
You can also manually write the excerpt by entering the summary of an article in Excerpt field on Post Edit screen.
First, activate the post summary by going to Setting - Reading and setting the "For each article in a feed, show" to Summary.
The excerpt field can be activated on the Post edit screen by clicking on the "Screen Options" and making sure the Excerpt option is checked.
Activating the excerpt field
A new field will appear below the main editor where you can write the excerpt for each post.