Show only part of a post?

I have a question about how to show only part of a post:

If you have a really long post, how do you make the post only show like half of the whole thing on the main page? I can’t find anything on the settings page, and I can’t seem to get it working.

If you can only see about half of it and there is a read more button, that would be good.
Any ideas?

JEFF