Defining your research question is first step in preparing to do a comprehensive review.
Step 1. Write out your question as a question
Example 1: In middle aged men, what is the effect on body composition of resistance training versus ketogenic diet, alone?
Step 2. Make sure that your review is needed. Check to see if a similar review has been done recently.
Do a simple search in Google Scholar or a relevant database (Medline, Sport Discus, PsycInfo), to see if there are recent reviews covering your subject.
Step 3: Use the PICO format (or similar) to identify the major concepts in your question.
P (Population) middle aged men
I (Intervention) resistance training
C (Comparison) ketogenic diet
O (Outcome) change in body composition