First of all, I recommend you to learn how to conduct a systematic literature review. Then, you should study quantitative methods to pool study results together (meta-analysis).
The PRISMA statement, the Cochrane Handbook for Systematic Reviews of Interventions and another book entitled "Introduction to Meta-Analysis" by M. Borenstein et al. are three "must-read" resources for beginners (and they are also very useful for consultation at any step of your study).
You may also take a look at our brief guide for clinicians:
Article Reading and interpreting reviews for health professionals: a...