To use the MeSH vocabulary (Medical Subject Headings) in MEDLINE:
Make sure you're searching only in MEDLINE.
Click the "MeSH" button in the blue toolbar.
Enter your first term in the browse bar and hit "Enter."
Check the box to the left of the term you wish to use or explore (this does the same thing as selecting the "Explode" check box). Clicking the small "talk bubble" next to a term will pull up scope notes explaining what information is grouped under this subject tag.
If you're adding more terms, click the "Browse Additional Terms" link underneath the subject list and repeat the above steps to select each additional subject.
When you've selected all of the subject headings you need, check either the "AND" or "OR" button beneath your selected search terms. Then click the green "Search Database" button. If the initial search doesn't do what you wanted it to, you can edit the text in the search bar if you need to add parentheses, change an "AND" or an "OR," etc.