Before you start with a business plan you need to figure out if you have a business and then if you need funding for it.
If you need funding you might need a formal business plan in order to tick a box. If this is the case I would simply make use of an app called Stratpad. It it easy to follow and will give you a pretty decent business plan at the end
If you are not looking for investors then I would simply just go through something like the Business Model Canvass to give yourself a course to follow.
Not sure what business you are in but regardless this might be an appropriate quote. "A good plan executed now is better than a perfect plan executed next week." General Paton