A great screenwriter once said, "When in doubt have a man come through a door with a gun."
But seriously, you're probably getting way ahead of yourself. With a new idea you need to take at least a month to work on structure and character before you write word one. You have no ideas because you don't have the characters. You may have characters in mind, but that is not the same thing as models of humans with hopes and dreams, motivations and desires, and backstory. The first hundred pages we write always winds up being backstory anyway. An idea does not a story make, regardless of the emotional charge you get when you think about it.
Yes, it is tempting to say to ones self, "It works better for me if I just write. Forget all that boring stuff." If you're one of those, and you may not be, then clearly it is not true because you are stuck. The real reason we all say that, including me, is because quite honestly, the word smithing is the easy part. But Structure, plot, and characters are hard. Yet like with anything, if you do the background work, it will make the foreground work much, much easier.
If this is really a novel worth writing, and you are serious about it, two or three months spent fleshing out the characters and the plot should not be daunting in the least. We would all like to be Mozart and just sit down and write a symphony. Unfortunately most of us aren't Mozart. In fact all of us aren't Mozart. And no one else is either. Because only Mozart was Mozart.
Do your background work and after you've got everything set up, you will find the writing is really exciting, moves right along, and the plot stays tight and focused. And best of all, your characters behave. They aren't always off doing their own thing adding unnecessary page count to what is ultimately supposed to be a product that sits on a store shelf. Right.
So forget about writing for now and figure out what this story is all about.
-MM