Build and iterate the idea, and your understanding of the type of person that would want to buy it. Make each iterative step as you gain enough confidence to invest the time and money needed to take that step.

For instance, you can start by asking friends and family while it's still just an idea. If it seems to have promise, maybe make a landing page for it for free on somewhere like Weebly.com.

Start showing the landing page to friends and family again, but also to strangers now, and ask their opinion. Iterate the landing page, and prices etc. based on this feedback.

Maybe then build an online survey that you can circulate to more anonymous people within your theorized target demographic. See how they respond, and iterate the idea, website, and target demographic if needed.

Then see if you can get people to actually sign up. Contact everyone you've talked to up until now and show them the now polished website, idea, etc. and offer it to them at an early bird discount. See if anyone bites.

etc.

Let me know if you'd like more specific help with any of those steps, with regard to your specific idea,

best,

Lee