Prior to starting a systematic review you need to find out if a systematic review has been done or is under way. Checking registers, such as PROSPERO, is important in ensuring that you aren't repeating a review that has been done or registered.
Published reviews also provide a useful starting point for identifying studies in your area of research.
Some databases focus specifically on indexing evidence-based medicine, systematic reviews, etc. e.g. Cochrane databases
You can also search the larger subject databases such as MEDLINE, CINAHL, PsycINFO and limit the search to "systematic review". Some databases have a filter or limit option to limit the search to 'systematic review'.
It's also useful to search the large multidisciplinary citation databases Web of Science and Scopus, particularly if your research is cross-disciplinary.
When using this Database it is best to select the MULTI-FIELD SEARCH Option