I am trying to set up my shopping cart and my merchant services provided an API but no transaction key. Where would I get the transaction key?

These are the instructions that Authorize.net are providing for obtaining the transaction key:

1. Log on to the Merchant Interface at https://account.authorize.net
2. Select Settings under Account in the main menu on the left.
3. Click API Login ID and Transaction Key in the Security Settings section.
4. Enter the secret answer to the secret question you configured when you activated your user account.
5. Click Submit.

The Transaction Key for your account is displayed on a confirmation page. You can read more on their online guide here.