Searching PubMed:
Searching ProQuest
Guide to using Scopus
Searching Cochrane
Web of Science searching
MedLine
Yale University's Harvey Cushing/John Hay Whitney Medical Library has an excellent set of video tutorials that will walk you through the steps of searching the literature for a systematic review.
"This series of tutorials covers the fundamental concepts and general procedure of searching the health science literature in a systematic manner. They will mainly focus on systematic searches required by a "systematic review." The goal of these tutorials is to ensure that your search is comprehensive, methodical, transparent and reproducible, so that your conclusions are as unbiased and close to truth as possible."