You need to modify the controller class to allow it output data in JSON format
/**
* !RespondsWith Json, Layouts
* !Prefix something/
*/
If you want to get rid of adding .json to the request URL - just leave Json as a single possible output:
/**
* !RespondsWith Json
* !Prefix something/
*/
See this link for the details
On the other hand, you are able to force the JSON output using Accept: application/json
header in your HTTP requests, so no .json word should be added to the request URL. However, you still need to enable the Json output in your controller class.