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.
Description - What product(s) or service(s) are you offering specifically? Are any patents, copyrights, or trademarks needed? Have they been acquired/filed? What is the size of your business? Where will it be located? Will this require purchasing or building a facility? Will this require leasing a facility? At what cost? Has a lease been negotiated? What personnel will you need? Where will you find suitable employees? What equipment do you need? Will it be purchased or leased? What are the qualifications of your principals? How do their backgrounds promote the success of this venture? Why do they think this will be a successful venture? Possible Data Sources: local Chamber of Commerce; community colleges & local universities; local employee leasing company; real estate agents; US Patent & Trademark Office; US Copyright Office.
While these are easily available online, some of them are way better than the others. A lot depends on the approach that goes into writing the business plan.
When people think about getting into some kind of business, the first thing they should do are look for reputable small business consultants. These are individuals who have years of industry experience and can structure your organization in a manner ensuring positive results.