Find a competitor in the niche you want to go in. See what their site looks like, look at their users and try to identify a subset that you know about (like where they hang out, what they read, etc) and can reach easily. Then, create your product / service. Your site should do the basic functionality that the users want, but you need something that differentiates you / improves on the competition. Then, reach out to this group and get them to use your product. If you get to the point where you have a product/service and regular users who like it, then you are ready to scale.