PubMed has a single search box which allows you to enter your search terms as if you were doing a Google search.
Before you start your search, identify the main concepts of your research question and any limits you wish to apply. For example, if you were looking to find review articles published in the last 5 years to address the question:
How effective is aspirin in preventing heart attacks in elderly people?
Your search concepts and limits would be:
Enter your concepts into the search box and click Search:
Behind the scenes, PubMed automatically inserts AND between each word, which means it will only retrieve articles that include every concept you have entered. Your results are displayed below the search box.