The first place to look is the Cochrane Library (previously called EBM reviews). This contains several databases
With the exception of Cochrane Reviews, many of the references indexed in the Cochrane Library will be found using databases such as Medline, but the Cochrane Library only indexes studies which have been evaluated and found to be methodologically-sound.
We have short instructions available to get you started. This should give you a general overview of the subject.
Each subject area has several large well-established databases which should form the basis of your review. They are listed in subject tables, just follow the links below: