ISAE 3402 is the international standard for assurance reports on controls at service organisations, focused on processes relevant to their clients' financial reporting. SOC 2 is a framework from the American AICPA, based on the Trust Services Criteria: security, availability, processing integrity, confidentiality and privacy. Both come as Type I, covering the design and existence of controls at a point in time, and Type II, which also tests operating effectiveness over a period.
The report itself is issued by an external audit firm. An IT auditor you bring in helps you get there: a readiness assessment, building the control framework and system description, testing controls before the external auditor arrives and following up on findings.
Independence matters. Someone who helped design your controls cannot audit those same controls independently. We take that into account when we put candidates forward.