Our colleagues in the Bodleian Health Care Libraries have written a number of Guides to help.
The first guide will help you understand the way to glean the most out of the electronic resources. The second guide will help if you need to go in more depth into particular areas of medicine.
OvidSP particularly for medical journals in the strictest sense.
But don't ignore those looking at scientific journals more generally such as
ScienceDirect, Scopus & Web of Science