When you enter text in admin/Tools/Define Pags Editor, you can, if you want to, add in-line styles. It may even be necessary to do that some times. But most of the time, all you need to do is identify what the chunks of text are, not how they should look, when you're in the define pages editor. Then the styles in the stylesheet handle how they should look. For example...
If you want a sub-heading and a paragraph, paste the raw text into the Define Pages Editor then select just the sub-header text and click the Format dropdown (it probably says normal at first.) Select Heading 2 or 3 or whatever you want. (Generally, heading 1 is taken care of automatically by Zen Cart. It's your page title.) You don't have to define the paragraphs other than to separate them with a carriage return. Click the "Source" button in the top-left of the editor toolbar and you can see the result in code view. (This description applies to FCKEditor.) You almost never need to define font styles or sizes in the Define Pages Editor.
Now that you have the text entered, view the page in a browser. If it's fine, you're done. If not, then you want to go into your stylesheet.css file and edit it to style things the way you want them.
If you're not familiar with writing styles in the stylesheet, there are lots of places on-line to learn about CSS.
W3 Schools is often recommended.
In short, use the Define Pages Editor's buttons to define what the text is -- paragraph, heading, list, whatever -- and use the stylesheet to make it look the way you want.
Rob