Which makes this an excellent question. Indeed perhaps the most important question to ask before you build anything.

Too many programmers subscribe to the school of "if you build it through will come'. (Hint: that's not how it works.)

Firstly you don't have to show anything to potential customers, (and even when you have it, it's better not to show it.) You are selling benefits, not features.

Secondly, how to find them is exactly the hard problem you need to figure out (it'll be different for different products.) For me personally, I the late 90s / early aughts, I traveled to user groups (all over the world.) Turns out that "showing up" spread the word very quickly.

(That worked for me because it's a niche area and nobody else bothered to travel, so I stood out.)

The key lesson here- if you can't define your market, and if you can't reach them, you can't sell to them.

Every business has hard problems. This may be yours. Solve this problem first, not last.

2. Recognize a $space shortcoming that users would pay to resolve

3. Start building

Bonus points if it's an area you're passionate about! But don't expect everything you're passionate about to solve a marketable problem.