I've seen many experienced authors give a simple bit of advice: write down everything. They offer different reasons for this, but they all amount to the same thing. If you don't write something down, you might forget it.
There's a long file on my computer, called "untitled world notes", because I have no idea what else to call it. The notes themselves started as just a few pages of ideas about a world with a magic system I've never before. Yes, some people say every idea has been done, and the only difference is in execution. (Some won't even concede that last point.) I loved the world and magic I'd come up with, but there was a serious problem: I had no real story. Not yet.
That changed a little while back.
Inspiration strikes in the oddest places. I was at Dragon*Con with friends, at the Tolkien-themed event called "An Evening at Bree". My mind started wandering, and as I was already surrounded by elves, I started thinking of how to do elves differently. What I came up with fit perfectly into the world I'd created several years ago.
All of a sudden, boom, I had something greater - I had the start of a story. No, I had the start of dozens of stories. An epic series in this world started taking shape in my head. When I got back home, I sat down with the old notes file and started taking new notes. I fleshed out as much as I could of the elves, worked out their relations with the humans and some of their history, figured out how they deal with this world's magic. And somewhere in there, I realized something else:
Writing in this world, doing these stories justice and putting together the dozens of elements I want to incorporate, will take a long, long time. There's political and social manipulation going on that will take several books to play out. There are so many things to determine before I even know where to start. To put it simply, not only am I not ready to write this, I'm not yet good enough to write this.
It's a weird thing to realize my limitations like that. My confidence in my writing waxes and wanes, but this is the first time I've ever come up with something and had to admit I'm just not capable of doing this the way I want to. Yet. I have the seeds of the first few stories, I have an idea of where things will go, I have a few characters slowly forming themselves in my head. And I'm writing all of it down.
I don't want to risk losing any of this. The day will come when I'm able to put this whole puzzle together, when I can plan a dozen books in advance and let things play out as they should. When that time comes, I'll still have all the notes I've ever written for this story.
Lots of things will change by then, I'm sure. But I'm also pretty sure it will start with a morbid revelation followed by something blowing up. Seems like a good way to get things going.