I don't think I've ever come face-to-face with a perfect idea. A just-right idea. I've never had a book just pop into my head, fully formed. I've never been able to see the whole thing all at once.
My ideas are vague. They're fuzzy at best. They all pretty much look like the whole world does without my glasses.
I have to work at it. I have to take a chisel to it. I have to take this weird blob and smash it into something recognizable.
I have to make it become the right idea.
How does a blob become a good idea? Usually, it starts with character.
Quite simply, the story begins and ends with solid characters. Your main character will have an "arc." They'll start in one place mentally and emotionally, and they'll end in another entirely. The physical journey they'll embark on will facilitate this mental and emotional change.
Figure out who your main character is. What do they want? What do they fear? What (or who) stands in their way? What will happen if they lose? How will they triumph in the end?
As you can see, by answering these questions, you're already figuring out some pretty important plot points. It all hinges on character.
So don't be frustrated if your idea--the one you've been toying with to no avail--has been vague for ages. My books have all started out as vague ideas too. Every last one of them.
But I know from experience vague ideas can become refined. For me, the best way to start refining is by getting to know your main character.