There is more info in the developers corner at authorize.net about
integration in general. But here it is in a nutshell:
1. Install the nop shopping cart, and get it working up to the checkout
page. Use the latest version from http://www.nopdesign.com/freecart
2. Modify some of the script options, in particular, the OutputOrderTotal
option -- which MUST be set to 'x_Amount'. The other options can be set
to match your site.
3. In your manage cart page, have the form look like:
<INPUT TYPE=HIDDEN NAME="x_Version" VALUE="3.0">
<INPUT TYPE=HIDDEN NAME="x_Login" VALUE="your login ID goes here">
<INPUT TYPE=HIDDEN NAME="x_Show_Form" VALUE="PAYMENT_FORM">
<INPUT TYPE=SUBMIT VALUE=" Checkout Now ">
This form is very important, and will be your link into authorize.net.
You can read about options here at authorize.net's developers corner.
That's it! You should be able to pay now. There are more options, but
this will get you started.