How can I test my project using testFX

I'm a student working on a small javaFX project and I'm really new on testing a Software specially with GUI. I want to know how can I provide the method getRootNode with my rootNode from my GUI. I'm using testFX as framework

public class ButtonTest extends GuiTest {

@Override
protected Parent getRootNode() {
    return null;
}

}

public class MainApp extends Application {
 @Override
public void start(Stage stage) throws Exception {
    Parent root =    FXMLLoader.load(getClass().getResource("/fxml/PhotoOverview.fxml"));

    Scene scene = new Scene(root);
    scene.getStylesheets().add("/styles/Styles.css");

    stage.setTitle("Easy  Manager");
    stage.setScene(scene);
    stage.show();
}

/**
 * The main() method is ignored in correctly deployed JavaFX application.
 * main() serves only as fallback in case the application can not be
 * launched through deployment artifacts, e.g., in IDEs with limited FX
 * support. NetBeans ignores main().
 *
 * @param args the command line arguments
 */
public static void main(String[] args) {
    launch(args);
}

}