When you're writing a business plan in a situation like these, you need to address a few issues the intentional entrepreneur has already pondered. The first is do you really want this idea to become a full_blown business? Certainly it's flattering when you realize there's a market value for something you were doing anyway, but that doesn't always mean you should launch a business. A lot of accidental businesses form around fads or seasonal items, and may not be robust enough to function as year_round, money_making, enterprises.
A plan should include but is not limited to the following: 1. A statement of your business purpose 2. A description of your company 3. The goals of your company 4. The structure of the company (sole_proprietorship, partnership or corporation) 5. The product or service that you are selling 6. A market analysis of your product or service 7. Resources spent (time and money) 8. A financial plan to include financial statements 9. Information about the managing principals in the company 10. How you plan to manage and operate the company