Don't fool yourself by prematurely narrowing down on one type of customer. Reaching out to individual customers, segment by segment, is ridiculously time-consuming if you can let them find you instead.

Plant a flag.

Find a media channel where many types of your customers go, and make yourself and your intentions visible there. Hacker News, specialist forums and industry magazines are great for this. I found my first geographies, hundreds of collaborators and tons of useful tips with with a single blog post on Medium.

If you aren't sure who your first customers will be, this is one of the fastest ways to find out.