If you are using Authorize.net as your payment processor, there are a few things you need to do in order to get it working with your zen cart store.  If you need to set up an account, you can get one very quickly by going HERE.

1.  Log into your Authorize.net account

2.  Click on the ACCOUNT tab towards the top of your screen

3.  In the second section entitled SECURITY SETTINGS - click on API LOGIN AND TRANSACTION KEY - follow the steps to create these 2 items.

4.  Log into your zen cart store admin

5.  Mouse over Modules, then click on Payment

6.  Click on Authorize.net AIM (NOT SIM) and click the button to install the payment module.

7.  You will now see some boxes that will need your info from authorize.net - copy and paste your API Login and your Transaction Key from authorize.net into the appropriate fields

8.  In your zen cart admin, there is a box that asks for your hash code.  Type any code you would like in this box.  

9.  Read over all the other specs of the authorize.net payment mod and change things as necessary.  EG.  Do you want authorize.net to send your customer a email?  They will receive one from your cart.... so you may want to avoid confusion and NOT have authorize.net send one.  On the other hand... you probably WILL want them to send YOU the email.  Also.... be sure to click on Authorize/capture = TRUE.

10.  If you are ready to start processing orders, make sure you indicate that you are in production mode, not test mode.

11.  Save your authorize.net settings in the zen cart admin.

12.  Back to your authorize.net account.... click on the ACCOUNT tab again, then click on the MD5-HASH link and enter the code you put into your zen cart store here.

13.  Then click on the ACCOUNT tab and under SECURITY SETTINGS, you will see a link that says TEST MODE - click on that and make sure that test mode is OFF

That should cover it!  You should now be set up to use authorize.net as a payment processor!

Keep in mind, that authorize.net will require you to have an SSL certificate installed on your website in order to proceed.  They will also require that you have visible contact information and a returns policy.