It depends on the specific context of what you're trying to build, but I'd say that one of the best ways to initially test if there's customer demand for your idea is to see if people are already trying to solve the problem you've identified through some other means. To give you an example:

Let's say you want to build an app that helps businesses pre-screen candidates for interviewing -- how do businesses solve the problem today? Well, the hiring manager / HR team might do their own research by calling people who know the candidate and asking for feedback. Or the business might hire a headhunting firm to do this for them.

If people are already be paying someone to solve this specific problem for them, it's a good indicator that your idea might be worth something (provided you can come up with a better solution).

However, even if people are not currently paying someone to get their problem solved (but the pain point still exists, and people are still coming up with solutions of their own) it might be worth evaluating to see if you can come up with a solution + a business model that might work.

I hope this is helpful and let me know if you'd like any additional feedback!

This is an interesting, complex question (which means you're already on the right track).There are many things I'd do if I were 18 again, but here are a few important ones:

1) Read more books on topics that interest you -- I used to dismiss books a little bit back in the day ("why read a full book, when an article is shorter, but can provide the same conclusion?"). However, I've realized that the best starting point for gaining knowledge in any field is to check out what the smartest minds in those fields have to say. The critical thing to do is to choose your books / authors wisely. The initial chunk of knowledge you acquire in any field will set the stage for how you think about things in the future.

2) Channel your attention into more productive paths -- when I was a kid, I remember that I could literally play video games for 10 hours straight without realizing it. I now do the same thing, but for more productive things like figuring out my startup's strategy or learning to play a musical instrument. Channel your attention on areas that can add value to yourself and others and you'll soon be on track to becoming an expert in any field.

3) Plan a little bit and set some objectives -- If you start doing (1) and (2) above, you'll be better equipped to set concrete objectives that are aligned with something that interests you or that you'd like to accomplish. They don't have to be big, but they should be worthwhile to you and, if possible, be a little bit outside your comfort zone (but not too much). For example, a simple, but worthwhile objective could be "I'll go to the gym 3 times a week for the next month". The important thing is to hold yourself accountable for reaching your objectives, otherwise it'll be easy to give up if things get tough.

4) Go out and have fun, you're 18! -- This one I did do quite a bit of when I was 18, nut I wanted to mention it because it's very important -- your social skills and relationships are just as important (if not more) for your success in business and in life. So go out partying and meet new people!

The right answer for you won't be exactly the same as the above, but I hope this gets you going in the right direction!