I think Gornova is right, the best way to start is to implement basic stuff. Once the prototype is playable, you can add whatever you want. Keep in mind that this means you have to organize you work well: it is not simple even to make a simple game (even though it is a lot of fun, and that's the important thing of course!). It is particularly important to desing classes well, because when the game gets bigger it is difficult to modify it.
Since you'd like to add RPGs elements, I suggest you to take a look at Artemis entity framework: There is a section here in the forum about it. I personally find the entity-component-system approach great for games, even in small games I noticed how I can add/remove things easily, without changing much in the code. And I think it is particularly suitable for an RPG, because if you organize things well I assure you that every game object will be able to do anything you want with a very little effort. On the main page of Artemis there is a link to an article on the T=machine forum, which will explain you everthing about what an entity framework is. It is a series of 5 articles, but it's worth taking a look
