I think the first step is to ask yourself how your coupon business is going to differ from all the other coupon businesses that are out there. What problem does it solve for people that is not already being solved?
If you know that already then go out and talk to some business owners, explain to them what you are thinking of building and ask them for feedback on the idea. If they like it what do they like about it specifically. If they do not like it then what specifically do they not like about it. Once you have done that go out and talk to some people that you use coupons, and ask them for their feedback. At this stage you should know whether you have a good idea, an idea that needs tweaking to be good, or simply a bad idea.
If you still think you have a good idea after the above process then I would go back to the business owners (or whomever the people are that are going to be your revenue source) and ask them if they will buy your service if you build it.
If a reasonable amount of people say yes then you can start putting a business plan together and worry about raising money (which will be a lot easier if you already have clients lined up).
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