So what you would do in your controller is fetch your data, then put it into json, then return it. the code would probably look something like this:
// assume your model is called 'my_model'
data = my_model.find()
// generate json
// make sure to require 'json' in the controller and to include json in your extensions in your build.yml file
json_data = ::JSON.generate(data)
// return the string from the controller
render :string => json_data.to_s
and the response should be json. If you make a json request, the app should automatically know not to include the layout, but check the response to make sure.
Hope this helps.