Adding Authorize.Net
To accept payments through Authorize.Net, you will need to add them as a provider.
Getting an account
If you need to sign up for an Authorize.Net account, you can register for an account here.
Adding Authorize.Net to payFURL
First, log into your Authorize.Net account, go to Account (top right corner), then API Credentials & Keys (under Security Settings).

Take a note of the API Login ID, and generate a new Transaction Key.

You can use this to add Authorize.Net as a provider in payFURL. Go to Providers, New Provider and select Provider Type Authorize.Net:
