That's called a business plan. I can't help but recommend my free online book Hurdle: the Book on Business Planning, which is intended to take you step by step through the process. Also, Business Plan Pro, Windows software, which also takes you step by step. Don't reinvent the wheel.

Neither of these is going to tell you what to do. They'll get you thinking along the right lines, ask you the right questions, and help you with the mechanics. Every business is different, but the process of writing a business plan is useful for any business.

Tim