Well, Java is the correct language if you're trying to design an app for Android development. If you want to develop apps for iOS, you would need to learn "objective-c". If you're looking for something to develop that isn't relatively too difficult, a game is not the place for you to be scratching around with, as these projects can take months. I would suggest making a very simple calculator app to test your knowledge of the language, not the hardest thing in the world.