There is all sorts of material out there for learning how to perform audits. In the order of formal to informal: there are high-dollar
Lead Auditor training courses, there is the
ASQ Certified Auditor Handbook (also available in specialties like food safety and medical device), and there are how-to books like
Internal Auditing in Plain English - as well as so many others trying to simply explain it.
All of the expensive courses and certifications such as
ASQ and
Exemplar Global (used to be RABQSA) try to establish expertise and competence, but it isn't enough. Like Randy said, so much is about observing and doing. I have seen auditors with all those certification letters after their names and still been underwhelmed by their performance.
Good auditing requires a curious mind and ability & willingness to discuss a process on a level with the users, as well as the ability to analyze and pick away layers in order to understand what is really happening - without these soft skills, all that expensive training results in just certificates. Remember that the purpose of the audit is to verify effectiveness first, and in the course of doing so verifying conformance to requirements. This is assuming the process has been set up in such a way that is conforming to requirements. As you gain more skill you can learn to spot when the process itself has a gap in it.
My point is, effectiveness first and try not to worry about technicalities right now. Those can be taught but first the soft skills must be established so the technical knowledge does some good.
I hope this helps.
Disclaimer: I am an ASQ member and hold Exemplar Global certifications for Quality Assurance and Responsible Care auditing.