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.
The financial statements section should show the way things are now if you have an existing business, as well as a forward look at your checking account, or projected income statement. The only way a start_up company can provide an income statement and balance sheet is by projecting these figures based upon well defined assumptions. Both start_ups and existing businesses should include a statement of owner's equity.
Explain the fundamentals of the proposed business: What will your product be? Who will your customers be? Who are the owners? What do you think the future holds for your business and your industry?
It may not be pleasant to imagine all the "what ifs," but doing it now and planning for those unexpected events will improve your company's chances of surviving a storm. For an excellent step_by_step guide on the details of developing a risk management plan, see the article "How to Develop a Risk Management Plan," by Charles Tremper at wikiHow.com.