Unless their constitution says otherwise, any competent person can audit the books of, say, a local knitting circle or table tennis club.
But, to become an auditor of the type that examines and certifies the accounts of large companies, you need to start by getting a relevant degree, then training as a chartered accountant, then studying and qualifying further for membership of an appropriate professional body:
http://careersadvice.direct.gov.uk/helpwithyou rcareer/jobprofiles/JobProfile?jobprofileid=71 9&jobprofilename=Auditor
Chris