The National Library of Medicine (NLM/NIH) produces PubMed, the major biomedical and life sciences database; it's one of the best places to find high-quality research studies and is therefore highly utilized by all types of health care practitioners. PubMed, which incorporates MEDLINE, is HUGE (over 30,000,000 citations and counting), so it takes some practice to get the best results.
Searching PubMed:
1. Try to imagine how a researcher would describe your topic in choosing your search terms (keywords). This method is efficient, and can work well when you're not concerned about capturing all/most of the relevant research on a particular topic.
2. If you have too many results, set additional filters (limits); for too few results use additional synonyms or broader search terms. Note: Once you set filters (limits), they stay set in future searches until you remove them.
Watch this video about basic searching in PubMed, including how to use filters: