How do I get my first 100 users as a solo developer?
Short answer
Manually, one at a time, from places those people already gather. Find the five forums, subreddits, Discords or Slack groups where your users complain about the problem you solve, spend a fortnight being genuinely useful in them without linking anything, then tell people what you built. A hundred users is small enough that it does not require a growth strategy — it requires roughly a hundred conversations.
The longer version
The instinct is to look for a channel — a subreddit with 200,000 members, a Product Hunt launch, a viral post. Channels are how you get from 1,000 users to 10,000. They are not how you get the first hundred, because a channel only works once you know which sentence makes someone stop scrolling, and you cannot know that before you have watched a hundred people react to it.
So the first hundred are hand-made. You find the specific places where people describe your problem in their own words, and you spend real time there being useful before you mention that you built anything. Two weeks of answering questions with no link in your signature buys you the standing to post once about your product without it reading as an advert. Skip that and the post gets removed, or worse, ignored.
The part nobody warns you about is that this is slow and feels like failure while it is working. Ten users in a week looks like nothing next to a launch-day spike. But ten users you talked to individually will tell you why the other ninety bounced, and a spike of 2,000 anonymous visitors will not tell you anything at all.
The full argument, with the rest of the picture around it:
Why Developers Can't Market Their Products (And What Actually Works)