Expanding your general knowledge of the Java language and how OOP (object oriented programming) works should get you well on your way; and looking over what methods are available both within Greenfoot and some of the more useful Classes provided in the Java API would not hurt (for examples: the List class, Math class, and Color class). The Greenfoot API is accessible through the 'Documentation' link at the top of the page. The Java tutorials, trails, and API can all be found though Java tutorials index page. The second section on the left called 'Trails Covering the Basics' would be a great place to start; and the API documentation link is located in the 'Other Resources' box on the right.