Once you have settled on a research question, it's time to come up with inclusion and exclusion criteria. These will determine the scope of your review. Here, considering a variety of variables, you will decide exactly what kinds of studies will and will not be included in your review. The following graphic from the University of Melbourne's guide to systematic reviews identifies some common criteria.
Using these and other criteria, you can define exactly what will be included in your review. Do you want only randomized controlled trials? Are you only including studies focused on adults? This is the point at which you make those decisions.