Cochrane Training offer a free online learning resource with an accessible introduction to health evidence and how to use it to make informed health choices. There are four modules of up to 45 minutes each, covering the basics of evidence-based medicine, understanding randomised controlled trials and systematic reviews, and resources to help inform decision-making. It is free to access by logging in with an email and password (no other details are needed).
If you are looking for a short overview of the different research designs used to produce health evidence, this guide from the Alliance for Useful Evidence has an excellent summary and a quick pros and cons list of many research designs (PDF) (see pages 21-23). This is a good place to start if you are new to reading about health evidence.