About being competent, each organization has the authority to determine its competency requirements for its internal auditors. Normally, organizations consider that internal auditors should have knowledge of the audit criteria (ISO 14001:2015 in this case) and should have training in internal audits. You can even decide that an auditor has to study a book on audits or attend an online course and do an in-house exam. Internal auditor competence requirements can be established in a job description, for example. I would recommend training about ISO 14001:2015 and an internal audit course. As a plus I would recommend that you participate as an auditor, making part of an audit team, in 2 or 3 internal audits.
You can find practical information in the links below: