You are a software developer (so am I) so you are probably thinking of what you need to write. That's not as important in starting a business as you think, even if the business relies critically on software.
The first part of starting your business is to do some planning. This basically looks like "how we are going to make money"; but, it is done in a much finer grain detail than you are dealing with.
Let me give you an example, I came up with a brilliant idea on how to save money on heating water; and, I also had a pretty good idea of how much money it would cost to build the prototype. As I was working out my business plan, I realized that my plan ran directly in contrast with the law, because of a water heater health-related issues that I knew nothing about. If I skimped on doing a full business plan, I would have invested heavily in a startup that was guaranteed to fail.
An older example, I wanted to open a computer service shop. In doing that shop's business plan, I realized that small computer repair shops had very high overheads (cost of keeping the shop open) in my area, and that if I opened it, I would effectively be working 12 hours a day and earning less than half my (then) current salary.
Good business plans do more than detail who their customers are, they also detail how many of them there are, how many of them you're likely to attract in the first year, how likely you'll be able to keep them happy (support / service), and roughly what your expenses and income would look like. If you can't even estimate that, odds are you don't know enough to open a successful business. Yes, there are rare businesses that succeed without these details, but there are hundreds more that fail.
You probably know how to design your software. If you're opening a business, you need to put that kind of effort and dedication into designing your business too.