I saw Mario Bros game on Brent Aurelis tutorials on YouTube and basically he creates the game screen with implement screen, but I want to make my game start up with the main menu which I can't.
How can I do that?
Maybe just change file name of GameScreen to MainMenuScreen and set up your menu here ? This is just a question of naming, nothing forbids you to make your MainMenu in GameScreen and nothing forbids you to rename GameScreen class to MainMenuScreen.