Drupal - Authorize.net payment processing on site

I have a Drupal site where I will be selling access to information. When the client pays they will become an authenticated user. I am using Authorize.net as the payment gateway.
1. What is the easiest way to do this? What modules would I need etc...
2. We are also providing another service which will charge the client 99 cents per month for a newsletter. Please keep this in mind as well.

Thanks.