Before embarking on your systematic search, you'll want to define your research topic in an easily searchable way. This involves separating out the key concepts of your research question. There are various frameworks to help you do this.
In health systematic reviews, PICO is a helpful framework for clinical research questions. For other types of research questions, especially outside the health sciences, there are additional frameworks. For example, PCC for scoping reviews. The Frameworks for Research Questions from the University of Maryland Library provides a list of options.
PICO example:
The worksheet and slides below can help you formulate a searchable question.
Is there already is a recent, well-done systematic or scoping review on your topic, or has another team has registered a protocol indicating a review is underway? The databases listed here and others in your subject area or Google Scholar can be searched to find reviews or protocols. For ideas for databases in your field, consult a research guide or a subject librarian.
Some initial basic, exploratory searching is also a good idea to gauge how much literature there is on your research question, and what inclusion and exclusion criteria might make sense to answer it. Based on the number of results, you may opt to broaden or narrow your research question.