If your Authorize.NET account is set to be in test mode then you may need to use a test credit card to get the transaction to go through properly. If this is the case you should have this information available to your somewhere on the Authorize.NET site when you login.
If you have it set to be test mode from your account you should make sure that you do not have test mode enabled in the globals file in any way, they should be set to false here.
It sounds like you are just getting the regular text message on the failure page, to get a more meaningful response on this page you can use the response reason text binding to show the reason. If it is not clear then you can add in the full request and response bindings onto the page to get at all the details.
Here is the code for the response reason text, put this on your checkout failure page where you would like to see the message:
<?php echo(AuthNet_Result("Response Reason Text")) ?>
These next two bindings are for the full request and full response, you would also place these on the checkout failure page where you would like to see the messages:
<?php echo(AuthNet_Result("Full Request")) ?>
<?php echo(AuthNet_Result("Full Response")) ?>