I have no idea where to start when creating a class diagram. I am just drawing it out on a piece if paper so I do not need any special software at the moment. How do you know what all your classes need to be and how do you know what methods and fields/attributes that they are going to have? One thing to keep in mind (I do not know if this will affect any answers). This game I am creating needs to implement the strategy pattern, and it also needs to use the model view controller. Thanks a bunch.
When I diagram, I only draw out major parts of the application. I don't go down to the level of classes and methods very often, and certainly never when I'm first starting to plan out a design and architecture.
Also, with something like the Strategy pattern, I never start out with the intent to implement that particular pattern. With me, the Strategy pattern is always something I recognize after seeing certain aspects of my design evolve after I have gone through several iterations of test/code/refactor.
The MVC pattern is something you can have in mind from the start since it is general enough to actually be useful in guiding your high-level module design and architecture.