1.) Create an account and login to https://www.authorize.net/ ( or https://sandbox.authorize.net/ for test environment )
2.) Go to “Account > API Credentials & Keys” section
3.) Save the “API ID” for provided field, then, fill in the question to obtain new “Transaction Key”
4.) You can check the authorize test card here. https://developer.authorize.net/hello_world/testing_guide/