Download Truespace and Paint.NET (you can Google them), That should allow you to create any models you need for your game.
Before you start with DarkGDK you should really learn the C++ language, A great tutorial can be found here:
http://www.cplusplus.com/doc/tutorial/
I suggest you read and understand everything up to and including the classes section before you start really making anything with DarkGDK.
Once you understand the syntax make some console application and try and complete a number of different "challenge" programs (can Google these).
I don't want to dissuade you, but programming is a lot of work, you shouldn't expect to complete such an ambitious project as you described by the end of this summer, or even the next. It is going to take a while, especially if you are just learning C++ now.
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Currently 1500+ lines of code into an "over-the-shoulder" action RPG with combat based on rag-doll physics.