The structure, content, and format of your business plan will depend on your business idea and your intended audience. If you're trying to raise money from investors or borrow money from a bank, your business plan will have to present solid financial data and market research in a professional, polished package. On the other hand, if you're funding your business yourself, you can probably forgo the sales pitch (and the fancy paper), but the basic principle is the same: Do your homework and create a business plan that gives you a realistic picture of your proposed business.
For more information on creating a solid business plan, see Business Plan Basics. Another excellent Nolo resource, How to Write a Business Plan, by Mike MCKeever (Nolo), will show you how to write the business plan and loan package necessary to finance your business and make it work.