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 auditor, making part of an audit team, in 2 or 3 internal audits.
You can find practical information in the links below: