Which editor are you using? Are you talking about the line-height in the actual editor itself or are you referring to the content that you create with the editor?
I'm using JCE.
I have some articles in my website but they don't have the line-height that i want.
From now on, i want to create some article, and have a line-height per default.
"The smart woman is not one that have a big knowledge, but rather the woman with little knowledge do so much."
So, what you actually want to do is deeper than the editor. You can set the default line-height of every article on your site (including those which are already written) by adding the following to your template.css file (replacing 1.5 with whatever line-height you desire):