Android REST API connection

I'm a little bit to stupid - sry for that.

I wrote an API which gives some JSON back. My goal is to use this API from an Android App. I've tried it with AsyncTask but failed hard.

I imagine to use it like this:

• Calling the class, telling the URL and the type of the result. (which json, like the account information or some data)
• When the loading is finished, calling the right class of the result, with the result as argument.

Here's the link to the API: Link

What do I have to do?

EDIT:

Here's my code now. He doesn't like the GetRequest variable type, also getHttpClientInstance is not possible. He also cannot resolve the method execute on MyAsyncTask.

@Override
public boolean onOptionsItemSelected(MenuItem item) {        
    if (item.getItemId() == R.id.action_settings) startActivity(new Intent(this, editPreference.class));
    if (item.getItemId() == R.id.action_kurse) startActivity(new Intent(this, Login.class));
    if (item.getItemId() == R.id.action_refresh) {
        String url = "http://vplan.florian-schmidt.org/api/get_klassen.php";
        new MyAsyncTask().execute(url);
    };
    return super.onOptionsItemSelected(item);
}

class MyAsyncTask extends AsyncTask <GetRequest,String,JSONObject> {

    @Override
    protected JSONObject doInBackground(GetRequest... params)
    {
        JSONObject data = null;

        GetRequest eventRequest = params[0];
        if (eventRequest instanceof GetRequest)
        {
            DefaultHttpClient httpClient = HttpClient.getHttpClientInstance();

            try
            {
                HttpGet httpGet = HttpClient.getHttpGetInstance();
                httpGet.setURI(eventRequest.getUriString());

                httpGet.setHeader("Content-type", "application/json");

                HttpResponse httpResponse = httpClient.execute(httpGet);

                //Check is authentication to the server passed
                if (httpResponse.getStatusLine().getStatusCode() == 401)
                {
                    // do some actions to clear userID, token etc ...
                    // finish
                    finish();
                }

                HttpEntity responseEntity = httpResponse.getEntity();

                if (responseEntity instanceof HttpEntity)
                    data = new JSONObject(EntityUtils.toString(responseEntity));

                responseEntity.consumeContent();
            }
            catch (ClientProtocolException CPException)
            {
                //set data to null, handle and log CPException
            }
            catch (IOException ioException)
            {
                //set data to null, handle and log IOException
            }
            catch (JSONException jsonException)
            {
                //set data to null, handle and log JSONException
            }
            catch (URISyntaxException useException)
            {
                //set data to null, handle and log URISyntaxException
            }

        }
        return data;
    }

    @Override
    protected void onPostExecute(JSONObject jsonObject) {
        TextView tv = (TextView) findViewById(R.id.textView);
        tv.setText(jsonObject.toString());
    }


}