I'm not sure exactly how much you understand of the world of C++ and OOP, so I'm just going to suggest a way you might want to look into to get you going with the GDK and your 2D RPG.
I think that the best approach to the situation is to sit down with a good foundation book on C++ to teach you at least the very basics. I started out with "SAMS Teach Yourself C++ in 24 hours" which I grabbed for a few pennies a while back off ebay. A good investment I might add, for me anyway.
The next thing you should do is work your way through the tutorials included with the DarkGDK installation, and while they do concentrate towards the 3D functionality, it'll give you a feel for what the GDK will be like to work with.
From there I'd suggest looking through the documentation on the commands available to you (there's a help file with the install explaining the commands), and as you're looking at 2D games you might want to play around with the Image and Sprite commands, doing a few experiments with them. You can also always use tutorials designed for DBPro users, for example, in the codebase section of the website, since although it's a different language, most of the commands can be translated easily between the two (e.g. "Sprite" in DBPro is the same as "dbSprite()" in the DarkGDK). Finally when you're in doubt about a specific way of tackling a problem, the forum's the best place to ask.
I must go now, it's like 1:00am and I've got work in the morning, so I'll see if I can dig up some tutorials later.
