Having got a fantastic idea, the next thing to consider is how to plan your story.
There are lots of different methods here, and the first thing I'll say is that different things work well for different people. There's no right or wrong - and some of the best authors I know will just scribble down a couple of notes, then write everything out chronologically, without having a plan. For them, this seems to work well. However, if I try doing something like this, I get ridiculously lost.
The first thing that I would do, and that I would recommend doing, is making a mindmap. Using a free program like x-mind can be particularly good for this, as it allows you to just add on further sheets, use different colors, and generally make everything pretty. Alternatively, grab a whiteboard or a pen and paper, and just put down all the ideas that you have. Neatness does not matter at all, just get the things down - you can always cross stuff out afterwards, but if you forget an idea at this point it might be gone for good.
Now that you have a scribbled page covered in notes, you need to try and turn it into an actual story. Again, different methods work for different people, and you can decide if you want to plan things out chapter by chapter, or go for a more overall story shape.
If you have no idea of where your story is going at this point, it's worth taking a look at different story structures, which you can adapt to meet your needs. We're all familiar with the fact that a story needs to have a beginning, middle and end, but that is often not useful for more complicated stories. I would recommend the five point structure and the seven point structure as starting points - picture it like a roller coaster, with various raises and falls in action - a constant rate throughout will either be exhausting or boring for readers.
I would also suggest taking a look at the snowflakemethod in which you slowly built up the story, going from a single sentence, to a paragraph, and then expanding into higher level details, describing characters and a plot, creating a synopsis. I find some of this works for me, but I don't go through all the steps, as I don't like having absolutely everything detailed before I start writing - if I know what's going to happen before I write, why would I bother to write it?
One final method I find quite helpful (if a little intimidating at first) is using a spreadsheet (or a table) to keep track of different subplots or characters - just going through chapter by chapter, and seeing what is happening in each plot/to each character.
When you've got a plan for your story, be it a several page synopsis or a half page full of scribbled notes, it's time to start writing, and that is what I will be looking at next week! See you then.