Can you elaborate on your issue more (screenshots of anything you have in place are great)? I'm assuming you are trying to get Marketo data into another service via Rest API, correct?
Here is the marketo RestAPI documentation: http://developers.marketo.com/rest-api/
Please confirm and if you are able to tell us the name of the service you are using it would be helpful as well.
Always works that way by default (it's in the docs). Specify fields with the `fields` property of the call.