Authorize.Net

Go to the Extensions → Payments → Authorize.Net 

Status: - MUST be "ON" for Payment module to appear in your store.

API Login ID: - Enter your Authorize.net API login ID, which can be found in the Account Settings of your Authorize.net account.

Transaction Key: - Enter your Authorize.net transaction key, which can be found in the Account Settings of your Authorize.net account.

Public key: Enter your public key.

Test Mode: Select whether or not the Orders placed using this payment method will be test Orders. Before you take your site live you will want to make sure you set this drop-down to Test.

Settlement Type:  Authorize.net is an online payment method. Therefore, the credit card must be Authorized by the payment gateway when the order is submitted, in order for it to complete. However, you can choose whether you want to Authorize Only or Capture the payment simultaneously during the Order creation. If you choose to Capture, an Invoice will automatically be created for the full quantity of items in the Order.

Success & settled Order Status:  Select the Order Status of orders created using this payment method.