On Payment Plugins management screen, click on os_authnet payment plugin to edit it. The following parameters need to be configured:
Sometime, your Authorize.net might not work well in Test Mode. But it does works in Live Mode. In that case, if you want to test it in test mode, you can use the following Authorize.net account for testing purpose:
If you have recurring subscription plans, you will need to set Slient Post URL of your authorize.net account to http://domain.com/index.php?option=com_osmembership&task=recurring_payment_confirm&payment_method=os_authnet. Of course, you need to replace domain.com with your site domain. You can follow the instruction at https://support.authorize.net/knowledgebase/Knowledgearticle/?code=000001399 to setup silent post URL in your Authorize.net accocunt.