The terms AND, OR, or NOT can be very useful. They're called Boolean operators, and they allow you to do a more complex search.
For instance, if you need articles on heart attacks, you can use the search term "heart attack." However, you'll find more articles if you search for "heart attack" OR "cardiac arrest." This search will pull up any article that contains either "heart attack" or "cardiac arrest."
Sometimes a good way to narrow down results is to use the operator NOT. If, for instance, you're looking for articles on children 12 and under and you get a lot of results about teens, you can include NOT teens in your search. You can even combine operators using parentheses, such as NOT (teens OR adolescents).
If you're not finding what you need, brainstorm some synonyms for your keywords. The articles you're looking for might be out there, the authors might just be using different terminology. When searching for articles, you're trying to guess which words will be featured in the titles, abstracts, and other important areas of the articles you're looking for.