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.
Additional getting started information can be found in this New merchant overview webinar tutorial.
More training videos can be found here.