There are two types of audit: An internal audit is when someone within the company audits the QMS, while the certification audit is when the certification body sends auditors to ensure that the Quality Management System meets the ISO 9001 requirements. To become an internal auditor, you would need to complete an internal audit training course. To become an ISO 9001 auditor and work for certification bodies, you first need to pass the final exam in the ISO 9001 Lead Auditor course.
Once approved, you have to find a certification body willing to accept you as a training auditor, so you can accumulate audit hours working on certification audits under the guidance of more experienced auditors.
As you accumulate experience, you will receive more tasks and responsibilities, until you can lead your own team of certification auditors.