The best way to start writing your business plan is to just grab a pen and paper or sit down at your computer and put down some very basic notes in any free form manner that you choose. Then go back and keep refining your ideas until they sound great.
While a finished business plan will be a large document, you can more easily motivate yourself by dividing the project into its specific parts. Too many entrepreneurs get discouraged or lose their motivation simply because they feel that writing a business plan will take too long.
Remember, good writing is re-writing.