Search more than one source. You should be as exhaustive as you can given the scope, time and resources that you have available.
You're going to need to search specialist databases — Google Scholar just isn't going to be thorough enough. It can work well when finding single relevant papers but it is not ideal for literature searching for a review because:
the search interface is quite limited
it isn't clear what's being indexed which makes it difficult to report your searches
different users may get different search results depending on their historical use of Google
saving or downloading all results in one go is not possible.
Bibliographic databases are the best resources to use as they allow structured searches using both text and subject indexing, and allow you to save your search strategies and search results easily. Your selection of databases and resources will depend on your topic — be sure to refer to your Subject Guide and the eResources Guide.
In general a systematic review in the area of health and medicine should search MEDLINE and EMBASE plus another specific database related to the review subject.
Remember that there is a difference between databases (like MEDLINE and CINAHL Complete) and the search interfaces that host them (such as Ovid and EBSCO). Some databases may be available to search via the same search interface (for instance you can search MEDLINE, EMBASE and PsycINFO databases via Ovid) and then there is no need to search them via other interfaces.
Ideally your systematic review should cover published and unpublished literature in order to avoid publication bias. The unpublished literature is sometimes referred to as grey literature (see the links below). If time and resources allow, you should include such literature in your search.