So, although a detailed business plan may not be required for an online business, I am going to include it here so you can at least look at and consider each section and determine yourself if it applies to your business.
Do not try to write your business plan in one sitting. As I mentioned in the beginning, for a traditional brick and mortar business, it could take in excess of 100 hours to compile all of the information needed into a comprehensive but yet understandable document. For online businesses, probably not that long. But your final product should be well thought out, well documented and easily understandable.
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.
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)