To create a single post you have to:

If needed you may also fill in additional fields on the right:


IMPORTANT: In order to display short post description on your blog, category, archive pages you need to activate Excerpt under Screen Options (see point 7). Fill in the excerpt field that appeared on the page below:
Or you can use tag Read More tag in your Visual or Text editor. The text written before this tag will be shown on your blog, category, archive pages.
OTHERWISE The entire post content will be displayed on your blog, category, archive page