Your Authorize.net merchant account credentials are a Login ID and a Transaction Key. Pepperi requires this information to connect to your account and send transactions.
These links will explain in detail how to obtain this information, and how to use your Authorize.net account in Test mode, before going live with ePayments.
Once you have completed testing, switch your merchant account to Live mode and deselect the "Test mode" option in Pepperi's ePayment settings in your Transaction Type.
A video tutorial explaining test mode can be found here:
More useful video tutorials from Authorize.net can be found here.