Writing a business plan is not a case of knowing where to start but of knowing where you want to end up.
A business plan is a tool to achieve your goals, which should be written with the end in mind. You might want to encourage investment, apply for a loan, as an aid to strategic planning, staff planning or for something else but a business plan is useless unless a) it fulfills its purpose and b) is regularly updated.

Essentially, there are three core elements to a business plan: -

For a business to succeed, three elements need to be managed passionately – product development, marketing, and financial management – which I call the trinity of management.”

- Ernesto Sirolli, Sirolli Institute

So your business plan should at least have these three areas of Product Information, Marketing and Finance and they should feed into one another to form a cohesive plan of action.

Subsequent posts will deal with each of these topics in turn.