@gloriaashby
t's important to know that Blog editors and wordproccessers are not the same and do not work the same way. The web standard is one blank line only, so you cannot add more blank lines by pressing return repeatedly like you would do with a typewriter or in a word processing application. If you want to learn more about how to format and space your posts and pages see:
http://en.support.wordpress.com/writing-and-formatting-poetry/

As houstonweaver has said, if you are working in the Visual editor, to get single spacing between paragraphs hold the SHIFT key down and simultaneously click ENTER at the end of the paragraph.

If you are working in the HTML (Text) editor, a single click of ENTER will give you a single-spaced line, and two clicks of ENTER will result in double spaced lines. http://onecoolsitebloggingtips.com/2011/09/13/wordpress-formatting-and-spacing/

Spacing

Hitting the Enter (Return) key will end the paragraph you are writing and start a new one. If you want to start a new line without creating a new paragraph, press the Shift key while hitting Enter.

Check out this support document, http://en.support.wordpress.com/visual-editor/