Hi Writersmirror, welcome to chrons!
Here's another possible path.
Learn a bit more about writing first - do some short stories, not necessarily connected with your idea, but they could be. Force yourself to write to a word limit in these stories, say 2,500-3,000 words. Use these to work out how to do beginnings, middles and ends. (Honestly, 2.5k words max can be quite difficult - but, I find, a nice chapter length, and chapters need structure too...!) This is also about learning how to finish things, as 3000 words are not going to take ages. This is invaluable experience.
Then...
(Always keeping your idea in the back of your mind, of course. Always write down any further ideas that make it bigger.)
...Perhaps you're a 'pantser' so you need to find a beginning and just start writing. You may get many ideas on how to start. Maybe you get a flash of genius and you are very excited about one of them. Well just get stuck into that one and follow your nose with the tale. If it fails, then you've got some material you can perhaps use again, or, if you will never use it again, it's useful practice. Don't start one if you aren't excited.
Or perhaps you prefer to be a 'plotter'. So write out the story, chapter by chapter, or even scene by scene. Roughly at first. Just a line per scene at first - you can expand it later with more detail. If you don't have a beginning perhaps start at the end and work backwards? Then when it makes sense to you, just expand out these scenes and chapters (any way you want. I tend to go from chapter 1 and sequentially henceforth, but there's nothing stopping you jumping about if you have a plan!)
Hope that helps