Try to put your idea into writing in no more than a few sentences. This might be quite difficult at first, but keep trying. Refine your idea as far as you can. In effect, this is the start of your business plan.
Once you've succeeded in explaining your idea in just a few sentences, you'll notice that it becomes a lot easier to write the rest of your business plan, as it provides sharper focus.
You can also give it to others to read to get tips and feedback.