if you have been copying and pasting do this first please:
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Then if you are copying and pasting from WORD see here please:
http://en.support.wordpress.com/posts/#publish-a-new-post
Microsoft WORD is not web compliant code and when copying and pasting text from WORD we must use icon icon 6 in Row 2 of the visual editor > http://en.support.wordpress.com/visual-editor/#pasting-text
Hopefully, the information linked to below may help you:
(2) 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/
See also > http://en.support.wordpress.com/writing-and-formatting-poetry/