in order to register a business, you do not even have to have a business plan, even though it certainly is a good idea to have given the business side some thought. Whether you are a sole proprietorship or any other form is not relevant.
But the moment you enter into any commitments that constitute ongoing financial obligations on your part, a business plan is a must, and at this point in time you need to have some projections about your revenue stream. This will be even more necessary if you need to borrow money from a bank to make investments.
I wish you good luck in your business.