Start with HTML, learn basics, what are tags, attributes, how to create paragraphs, headers, how to create links, add images. Don't bother with details, then after you learn these basics, you can start CSS.
Also, read comments in every lesson, they are really helpful.
The best choice is to start with HTML. Learn the fundamentals, try some practice then move to CSS. That should do it
Also, make sure you've got a good grasp on one concept before moving to another..
If there's any tough nut to crack in the lessons, use the Q&A forum and the comment section too...... Good Luck