There are great templates for this already: The YC application, Amazon’s internal press release, and Sequoia’s Writing a Business Plan. I found myself mixing and tweaking these templates because they don’t exactly match my model of the world, so I wrote up my own list.
I use this list both to develop ideas and filter them. If you adopt it, be careful about using it as a filter. Remember that in the early stages, good ideas are very easy to kill.
A great starting point to validate whether the idea you are formulating in your mind is worth pursuing.