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.
You may not have thought much about your competition or outsourcing some of your work, but things like that will impact your ability to make a profit. And you will find this especially so in the beginning phases of your business. Even you are just opening a lemonade stand in the front yard, you will still need to know what Susie is selling her lemonade for on the next street over!
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.
Make it enthusiastic, professional, complete, and concise. If you are applying for a loan, state clearly how much you need and be precise in how you are going to use it. Also include detail about how the money will make your business more profitable, thereby ensuring repayment of the loan.