1. Create account at authorize.net

2. Go to plugin settings in your admin panel and fill the following fields:
Login ID, Transaction key, MD5 hash.
All this information can be obtained from your Authorize.net account.

3. Specify Silent Post URL in your Authorize.net account.
Use the following url:
http://www.your-site.com/billing-authorize/silent-post (www.your-site.com here should be replaced with your own site domain)

4. If you are planning to use Advanced Integration Method, you need to purchase and install SSL certificate to your server and make sure your site is served over HTTPS protocol