There will be a lot of learning and adapting, but it should happen in a staged approach; much of it before actually 'launching'. Launching generally means exposing your full product/site/whatever to the entire general public. Before doing that you should do smaller scare, more targeted testing.
Before launching, but after you have a prototype / MVP, you need to gather data on whether people want to pay for it. There are many ways to do this before a full launch. The methods vary depending on the type of product. Without further description of your product I can't help you further in terms of those methods. Send me a message if you'd like to discuss it further,
best of luck,