This was originally a question as I was getting an error. After having downloaded the RestServices module, go to project settings -> runtime and change After startup microflow to a new microflow. Add StartPublishServices to the microflow. Add the service overview to your administration and compile. Create two modules. One normal one and one for REST. Create a domain model for both . -- Run Mendix, add a new Published Service and name a microflow to run for your GET service. Now in the microflow, have the entity type you want to get as the parameter and as the return object. Then run Postman and set http method to GET, the url to http://localhost:8080/rest/<service_name>/1 and press 'send'. And if all went correctly, it returns a json object with "Success": true".
It seems you have no object and you are calling a microflow without any object. Since you state you have nothing yet and want to test if your microflow works you must have an object with which you call the microflow if you use an object as parameter.