With Authorize.net, once you get your account you need to login and obtain 2 pieces of information which you then need to place into your website's backend under eCommerce -> Payment Gateways.
These are the API Login ID and Transaction Key. To obtain these details you need to go to Settings -> and click on API Login ID and Transaction Key in Security Settings section.
You then need to answer the security question to get the Transaction Key.
Once you have these details you need to go to eCommerce -> Payment Gateways on your site, select the country and Authorize.net from the dropdown lists and enter the information into the corresponding fields.