I know its been discussed like a billion times, and I have read a couple of questions/answers and this one in particular seemed like a good example -> example. So now I have tried to recreate the code and added my getParams() as well as my getHeaders().
Awkwardly I get a HTTP Status Code 400: 
E/Volley: [303] BasicNetwork.performRequest: Unexpected response code 400 for http://10.0.2.2:3000/classes
Since I have created the REST API I can see where this status code 400 comes from, its my NodeJS response if the req.body doesn't contain mAtt, mDatum, mRID, mVon. So now I know that my POST request is not properly working even tho I set my getParams()as well as my getHeaders() ... 
Now, my guess would be that I'm setting Params but I'm fetching req.body.mAtt, req.body.mDatum , req.body.mRID, req.body.mVon, that would explain why my NodeJS returns the status code 400. If I fetched req.query.mAtt I would probably get something back? 
Is there a certain method that need to be overridden by me, to actually set the body instead of the query params?
This is what my POST req looks like:
    JsonObjectRequest JOPR = new JsonObjectRequest(Request.Method.POST,
                       myAcitveLessonPOSTUrl, null, new Response.Listener<JSONObject>(){
        @Override
        public void onResponse(JSONObject response) {
            try {
                VolleyLog.v("Response:%n %s", response.toString(4));
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            VolleyLog.e("Error: ", error.getMessage());
        }
    }){
        @Override
        public Map<String, String> getHeaders() throws AuthFailureError {
            HashMap<String, String> headers = new HashMap<String, String>();
            headers.put("Content-Type", "application/json");
            return headers;
        }
        @Override
        protected Map<String, String> getParams()
        {
            Map<String, String>  params = new HashMap<String, String>();
            params.put("mAtt", "+1");
            params.put("mDatum", mDatum);
            params.put("mRID", mRID);
            params.put("mVon", mVon);
            return params;
        }
    };
    requestQ.add(JOPR);
Thank you!