Would like to learn how to get rid of double spacing in blog.

If you want single spacing, you can simply hold the shift key and then hit return.
That will give you single spacing. This also applies to word document and all types of word processing.

Otherwise, you can modify the stylesheet. Just look for the p tag and change the margin or padding. I see that you are using the Twenty Twelve theme. If you change the stylesheet, you should first make a child theme.

thanks.