Now that you know what problem you want to start to solve, you can do some research to find out if this idea would work on the market.
If you are just building a project for yourself, your market research is yourself. So you can skip this step, but you can also try to do this exercise with yourself to make sure you are building a solution that will solve the problem in a way that will work for you. The goal is to use the software once you build it.
Come up with a list of questions you could ask to figure out if the problem you are solving is worth solving, and how it should best be solved.
This is how you will begin to build project requirements. Your narrowed problem statement is the guardrail, and you need to fill in requirements that stay within that guardrail.