The databases you choose to search will vary according to your search topic. You should consult a number of different databases and conduct manual searching of grey literature and trials registers in addition to databases searches. For example, a clinical psychology systematic review topic may search APA PyscInfo, Medline (PubMed), Web of Science and APA PsycExtra. Consult the A-Z databases list to find databases for your subject.
You will have to modify and test your search strategy to ensure that it retrieves relevant studies, so good documentation is essential.