CINAHL and PubMed are your gold standard databases. CINAHL focuses more closely on nursing literature and PubMed covers medical literature more widely. You should start most searches with CINAHL or PubMed. Web of Science covers an even wider range of subjects, from business management and social work. The others are specialty databases, so read their descriptions to see if they're right for your search.
This tutorial was created specifically to help nurses efficiently find literature using PubMed. Its concise, targeted content consists of five videos with exercises to test your knowledge. The tutorial was designed to be completed in less than 30 minutes. (From National Library of Medicine)