Hey Zdenka; if you are committed to learning HTML and CSS, the best way to start is with the freely available courses at a website like CodeAcademy (Bing it), or with the relatively cheap classes offered by Treehouse or Lynda. With that said, it might not be a good investment of your time depending on how often you're going to need to use these skills.
Beyond that, you have a couple other options. There are some decent WYSIWYG editors like Kompozer that are a little more full-featured than Marketo's, and you can import the HTML from your template emails and then add the Marketo editable sections w/ the div tags. Just look into the HTML of your existing templates to see how the editable sections are labeled and designated.
Finally, there are some good templates available on Envato's marketplace. I've previously used the Fresh responsive templates as a starting point for my own customized email templates - but there are others as well.
Good luck!