If you purchased the Turbo Tax audit defense when you filed your 2015 return, log back into your 2015 return and the information will be there. You will be to call and speak to an audit defense representative.
If you didn't get the audit defense for 2015 the best thing to do is to answer the IRS and provide the information they are requesting. Some taxpayers who are being audited find that they need a CPA for the more complicated tax audits.