You may find that the easiest part is the actual writing of the plan. The real work comes in the data-gathering, which may take you a hundred hours or more, depending on what you already know or have researched. If your new venture is in an area where you've been working, you may already know about your customers, your suppliers, your marketing plan, your organizational structure, your financial and cash flow needs, equipment, inventory, and so on.
U.S. Census Bureau (census.gov): A source for a variety of useful statistics, especially the Economic Census that comes out every 5 years.
In Summary : Planning out your business on paper first gives you long-term benefits with potential investors, employees, vendors, and suppliers. The business plan becomes your roadmap to success, with pertinent data that shapes the course of your business start-up and lets you adjust your journey as contingencies arise. Business planning templates are readily available and data sources abound at your fingertips. You will achieve a solid understanding of your business as you work through each section of your plan.
Maybe it's okay as an assignment for an MBA class, but it would be just too confining and irrelevant for today's fast-paced business environment. Anyway, you're ready! You've thought about this business venture for a long time and talked it over with friends and everybody agrees it's a great idea. Best to strike while the iron is hot!