If you have to submit it to class, then you should create it. You can create a game like the one you linked. You should have learned some basics in class and using them, you can create the game. You could start by writing down what classes you will need and how they should interact. If you have any specific questions, feel free to ask.

Starting off with a big game is probably not the best idea. I'd recommend you to start with the basics of Java, and then slowly start making bigger and bigger projects. But a game already is not a good start I think

Yes, but one must first become familiar with the basics of the java language and what methods are available in the classes provided in the greenfoot package. Looking through the API documentation of the classes should help immensely in the latter, and the java tutorials should help with the former. Then trial and error programming will help in learning what is possible as well as how to accomplish things. This last part can be done while creating your game; but understanding how OOP (Object-oriented programming) works should be the initial step (the java tutorials can help there as well).