Don't paste from Word; you get all kinds of invisible madness that can screw up your blog badly.
If your theme double-spaces the text you can switch to a theme that doesn't do that. If you're hitting Enter at the end of every line, you will pretty much always get double spacing unless you hold down the SHIFT key when you do it. That will make it single-spaced.
If you want more formatting options than that, you may need to learn CSS and buy the CSS upgrade to make the changes you want. Do NOT buy the upgrade unless you know CSS, because it's not something you can figure out on your own and there's very little help in the forum and none at all from staff if you mess up.