I feel like writer’s gravitate towards two different kinds: the ones that just sit down and let their imagination take over their keyboard, writing while generating ideas and pushing on until they finish what they started, before moving on editing stage, etc (which can be very enjoyable), but then there are also the writers that can write like a paragraph or two, and realise that they have to do all the planning, research, characterisation, etc, before they can move on to the next point, and can have amazing, large plans but the writing can be a very slow process. The latter kind might find it disheartening to look at their two paragraphs of their new fantasy story idea, after diving into it with excitement, just to find that nothing comes to mind for them to write beyond that point. What we have are pants-writers, and plotter-writers.
Both ways are correct, but you always get something out of just sitting down and writing whatever comes to your mind, and enjoying it. And if you feel the need to make a plan, or study or research, then know that some writers are like this to an extreme (and it's not always beneficial to them).
In fact, I think that it really is a difference of being a Ne user (extraverted intuition) and a Ni user (introverted intuition). Ne-types are better at pants writing, and Ni-types are naturally inclined to be the majority of plotter-writers. That's just my take, being a Ni-dom and my experiences with different Ne-types.
So, now, your story idea needs to be built upon. I say work on your idea, the plan, brainstorm everything, and get the story ready for something like nanwrimo if that interests you, or, just start once you feel like you've got enough to go on. Typically: the start of the story, some of the middle, and the vibe of the story and characters. Enjoy!!