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.
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?
If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
Executive Summary _ A business plan starts with an executive summary, which is a one or two page summary of your business plan, or an introduction to your business. Although this section is at the beginning of the business plan, it is the last thing to be written. You'll be able to condense your business plan more succinctly once you have the opportunity to work through the other parts of the plan. The executive summary may be the only thing a potential investor or financier will read, so write it last because it has to be the most compelling.