The easiest way to implement any of the API's, inccluding AIM and ARB, is going to be with one of the SDK's built on top of the XML interface. In this case, PHP. If you want to figure things out, my advice is to download a fresh version of the PHP SDK (put it in a new location so as not to mess up the old one) and look in the docs folder for a file called AIM.markdown. This will give you fairly simple code for doing one-time charges. Now look for the file ARB.markdown. This will work for subscriptions. Note that ARB does not allow an initial charge - it starts on the first payment date, which has to be after the current date - so if you want to charge the first payment immediately and verify the credit card in the process, you need to use AIM to process that charge.
An alternative, and probably a simpler API to understand, would be CIM. Instead of setting up a subscription that just runs automaticallly, which means some amount of trouble if the credit card declines or the subscription fee changes mid-subscription, you set up a customer profile and billing profile, then have an automated process run every morning on your server that checks for charges coming due and generates them using the profile ID's. It gives you more control over the process and allows you to do both the initial charge and the subsequent charges with a single API, rather than having to combine AIM and ARB (plus a silent post page to pick up the ARB charges as they come through). Again, the best sample code is probably in the CIM.markdown file in the docs folder of the PHP SDK.
EDIT: Since AIM, ARB, and CIM (regular, not hosted CIM, anyway) all involve collecting credit card info on your site and connecting to Authorize.net internally to process charges, you do need SSL. That shouldn't be costing you much if you're buying from Network Solutions or Godaddy, however. And yes, the credit card info may be -transferred- through your site, but it's stored on Authorize.net.
