Start by writing a description of your business, including what stage of development it is currently in (conception, start_up, first year, mature, exit) and your plans for growth. Discuss the nature of your business, the main products and services you offer, the market for your products and services, and how and by whom the business is operated.
For an online business, a detailed and in depth business plan is usually not necessary unless you are trying to combine your online business with a traditional business. For most online business startups, the detail involved with planning a traditional business is not required. However, it would still be beneficial to you if most of the topics were still covered, even if only briefly. Having a written plan in front of you will help you to focus on important aspects of the business.
Financial Plan _ The financial plan consists of four sections: Financial Worksheets, Cash Flow Projections, Financial Statements, and Additional Financial Information. All of these components will tell the story of how you plan to start or grow your business from a financial perspective. It is vital that you explain the assumptions under which you have based your projections, for example, "We assume that there are no unforeseen changes in economic policy to make our products and service immediately obsolete." or "We assume interest rates will stay the same over the next three years." (both quotes from Bplans.com sample business plans)
I suggest that you construct easy to read tables and graphs for the financial portion of the plan. The worksheets suggested are: Salaries/Wages and Benefits, Outside Services, Insurance, Advertising Budget, Occupancy Expense, Sales Forecasts, Cost of Projected Product Units, Fixed Assets, Growth (or Start_Up) Expenses, and Miscellaneous Expenses. You may find some of the worksheet templates at PlanWare.org to be useful.