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.
James J. Hill Reference Library (jjhill.org): One of the nation's premier business libraries to bring you FREE and affordably priced tools and resources you can use to create a better business plan based on relevant and credible data.
Are there emerging markets or market segments? Where does this product or service currently perform best? Possible Data Sources: trade associations; trade journals; attorneys & accountants dealing with the industry; industry salespeople; state business websites; focus groups.
Internet Public Library - The Census Data and Demographics (ipl.org)/: An especially useful site that has links to information about countries other than the U.S.