This is a bit of an oldie but goodie from Lenny Rachitsky. Rachitsky writes about how great apps (mobile and web) got their first 1,000 customers. They all scaled mightily from there. But this article talks about their first crucial steps.
Just seven strategies drove initial growth (among the companies he focused on).
And, in all cases, precisely defining the target user was crucial. And then reaching out to them directly (in ultimately non-scaleable ways*) to learn from them and convince them to sign up made the difference.
*E.g. The founder going to a mall to talk to users is non-scaleable — but not doing it at first prevents companies from getting to scale over time.