CINAHL Plus contains mostly academic journals in various Nursing and Allied Health fields. You can search it using keywords by simply putting relevant terms in the search box.
However, you can really drill down your results to a small number of relevant articles by using the Subject Terms, otherwise known as CINAHL Headings. (These are also sometimes referred to as MeSH headings because they are modeled on the National Library of Medicine's Medical Subject Headings.)
The following video shows you how to get started in this process.