Depending on what kidn of auditing you want to do.
I have started as internal auditor, got the training with different companies for basic auditing skills.
Over the time I am now doing all 5 IATA disciplines (IOSA, ISAGO, DAQCP, IFQP and IDQP), IS-BAO, EU OPS, EASA 145 and FAR 91/135.
For IATA:
IOSA: Go to the IATA webpage to see all pre-requisists and other requirements.
ISAGO, DAQCP, IDQP: You must be sent by an airline
IFQP: Course can be booked privately, but working only for an airline
IS-BAO: Go to the webpage
www.ibac.org to look for requirements
EASA 145: several companies offer courses
If you have no experience, a good source for getting an aviation auditor training is, as one example, AQS in Frankfurt. Have good experience with them.