Got an idea? Good. Write it down. Write down every possible detail you have for this idea. Just have a sentence or less? That’s perfectly fine. It’s a start. Now depending on what you have written down, this next step might vary a bit for you. Do you have a character idea? Sketch them out! Bring them to life! Have an idea for a plot? Do the same thing; start sketching it out. Have a title idea? Start brainstorming some basic plot ideas that would fit under that title.
It doesn’t matter what you start with, it only matters what you do with it. Don’t feel disappointed, don’t feel defeated; everything will come together in time.