I help companies find their most profitable customers working with everyone from pure B2B startups to $75M+ consumer businesses. First, I would separate whether you are solving a problem for a consumer or a business (they have very different ways of being sold to).
To build on what Daniel is saying, I would begin with meeting as many people as possible you think you solve a problem for. At first, ask them what's bugging them about the area of their business or lives your idea pertains to. Is what they currently do too slow, too complicated, too expensive? Shoot for talking to a couple dozen people. Clarity and your friend group are a great place to start these conversations.
As you have these conversations you'll start to see patterns emerge. Holes in what's currently being offered in the marketplace, etc. From there you can start narrowing the conversations. After they've discussed what they don't like about what's currently out there you can pitch your idea. "Hey, listen it sounds like you really hate changing your summer tires to winters. What if I could have a team come to your home, change them and store the summer tires for you all for $49.99?" And the last bit is the most important part, asking for the sale. People will throw out objections: "Too expensive." "Nah, it's actually not that important. I'll change them myself." Or they'll say "yes" which gets some $ in the door. That's when you can start doing the traditional business planning of determining market size, etc.
I hope that helps!