Slide 3: Where do I find more subject headings?
If you find a good book or article that may give you inspiration for even more subject keywords to try, which you can learn by finding an article or book that looks relevant to you and reviewing the item's record for more subject keywords by clicking on the title of the article. See below the record for the "Education interventions..." article and the myriad of subject keywords you might try from it. (You will probably need to scroll down in the record.)
Even if one is not research the specific subject covered by the article, one might be able to find or "harvest" a lot of good subject keywords to try. We can see from the subject keywords for this article that it might even be possible to create a super-specific search using THREE subject keywords relating to our original research question - maybe Adolescents, Health Education, and Dietary Supplements? However, do note that such a search may be too specific. Sometimes using too many subject keywords together can actually be a bad thing relative to the information you are trying to find. It might be better to just search for subject: Adolescents and subject: Dietary Supplements, omitting health education, or you might try a keyword like "Middle School" with subject: Adolescents and subject: Health Education. Remember that it is up to you to help decide if what you find is relevant to your topic, and if what you're finding is not relevant, you need to adjust your search's subject headings, keywords, or both.
Remember to mix and match subject keywords and general keywords to find more articles, think critically about what you find and whether it works for your topic or helps answer your research question, and don't be afraid to consult a librarian if you need help deciding on, finding, or using a subject heading.